Makefile

# SPDX-License-Identifier: GPL-2.0
# trace-cmd version
TC_VERSION = 3
TC_PATCHLEVEL = 1
TC_EXTRAVERSION = 1
TRACECMD_VERSION = $(TC_VERSION).$(TC_PATCHLEVEL).$(TC_EXTRAVERSION)

export TC_VERSION
export TC_PATCHLEVEL
export TC_EXTRAVERSION
export TRACECMD_VERSION

LIBTC_VERSION = 1
LIBTC_PATCHLEVEL = 1
LIBTC_EXTRAVERSION = 3
LIBTRACECMD_VERSION = $(LIBTC_VERSION).$(LIBTC_PATCHLEVEL).$(LIBTC_EXTRAVERSION)

export LIBTC_VERSION
export LIBTC_PATCHLEVEL
export LIBTC_EXTRAVERSION
export LIBTRACECMD_VERSION

VERSION_FILE = ltc_version.h

LIBTRACEEVENT_MIN_VERSION = 1.5
LIBTRACEFS_MIN_VERSION = 1.4

MAKEFLAGS += --no-print-directory

# Makefiles suck: This macro sets a default value of $(2) for the
# variable named by $(1), unless the variable has been set by
# environment or command line. This is necessary for CC and AR
# because make sets default values, so the simpler ?= approach
# won't work as expected.
define allow-override
  $(if $(or $(findstring environment,$(origin $(1))),\
            $(findstring command line,$(origin $(1)))),,\
    $(eval $(1) = $(2)))
endef

# Allow setting CC and AR, or setting CROSS_COMPILE as a prefix.
$(call allow-override,CC,$(CROSS_COMPILE)gcc)
$(call allow-override,AR,$(CROSS_COMPILE)ar)
$(call allow-override,PKG_CONFIG,pkg-config)
$(call allow-override,LD_SO_CONF_PATH,/etc/ld.so.conf.d/)
$(call allow-override,LDCONFIG,ldconfig)

export LD_SO_CONF_PATH LDCONFIG

EXT = -std=gnu99
INSTALL = install

# Use DESTDIR for installing into a different root directory.
# This is useful for building a package. The program will be
# installed in this directory as if it was the root directory.
# Then the build tool can move it later.
DESTDIR ?=
DESTDIR_SQ = '$(subst ','\'',$(DESTDIR))'

LP64 := $(shell echo __LP64__ | ${CC} ${CFLAGS} -E -x c - | tail -n 1)
ifeq ($(LP64), 1)
  libdir_relative_temp = lib64
else
  libdir_relative_temp = lib
endif

libdir_relative ?= $(libdir_relative_temp)
prefix ?= /usr/local
bindir_relative = bin
bindir = $(prefix)/$(bindir_relative)
man_dir = $(prefix)/share/man
man_dir_SQ = '$(subst ','\'',$(man_dir))'
html_install_SQ = '$(subst ','\'',$(html_install))'
img_install_SQ = '$(subst ','\'',$(img_install))'
libdir = $(prefix)/$(libdir_relative)
libdir_SQ = '$(subst ','\'',$(libdir))'
includedir = $(prefix)/include
includedir_SQ = '$(subst ','\'',$(includedir))'
pkgconfig_dir ?= $(word 1,$(shell $(PKG_CONFIG) 		\
			--variable pc_path pkg-config | tr ":" " "))

etcdir ?= /etc
etcdir_SQ = '$(subst ','\'',$(etcdir))'

export man_dir man_dir_SQ html_install html_install_SQ INSTALL
export img_install img_install_SQ libdir libdir_SQ includedir_SQ
export DESTDIR DESTDIR_SQ

ifeq ($(prefix),$(HOME))
plugin_tracecmd_dir = $(libdir)/trace-cmd/plugins
python_dir ?= $(libdir)/trace-cmd/python
var_dir = $(HOME)/.trace-cmd/
else
python_dir ?= $(libdir)/trace-cmd/python
PLUGIN_DIR_TRACECMD = -DPLUGIN_TRACECMD_DIR="$(plugin_tracecmd_dir)"
PYTHON_DIR = -DPYTHON_DIR="$(python_dir)"
PLUGIN_DIR_TRACECMD_SQ = '$(subst ','\'',$(PLUGIN_DIR_TRACECMD))'
PYTHON_DIR_SQ = '$(subst ','\'',$(PYTHON_DIR))'
var_dir = /var
endif

# Shell quotes
bindir_SQ = $(subst ','\'',$(bindir))
bindir_relative_SQ = $(subst ','\'',$(bindir_relative))
plugin_tracecmd_dir_SQ = $(subst ','\'',$(plugin_tracecmd_dir))
python_dir_SQ = $(subst ','\'',$(python_dir))

pound := \#

VAR_DIR = -DVAR_DIR="$(var_dir)"
VAR_DIR_SQ = '$(subst ','\'',$(VAR_DIR))'
var_dir_SQ = '$(subst ','\'',$(var_dir))'

HELP_DIR = -DHELP_DIR=$(html_install)
HELP_DIR_SQ = '$(subst ','\'',$(HELP_DIR))'
#' emacs highlighting gets confused by the above escaped quote.

BASH_COMPLETE_DIR ?= $(etcdir)/bash_completion.d

export PLUGIN_DIR_TRACECMD
export PYTHON_DIR
export PYTHON_DIR_SQ
export plugin_tracecmd_dir_SQ
export python_dir_SQ
export var_dir

# copy a bit from Linux kbuild

ifeq ("$(origin V)", "command line")
  VERBOSE = $(V)
endif
ifndef VERBOSE
  VERBOSE = 0
endif

SILENT := $(if $(findstring s,$(filter-out --%,$(MAKEFLAGS))),1)

SWIG_DEFINED := $(shell if command -v swig; then echo 1; else echo 0; fi)
ifeq ($(SWIG_DEFINED), 0)
BUILD_PYTHON := report_noswig
NO_PYTHON = 1
endif

ifndef NO_PYTHON
PYTHON		:= ctracecmd.so

PYTHON_VERS ?= python
PYTHON_PKGCONFIG_VERS ?= $(PYTHON_VERS)

# Can build python?
ifeq ($(shell sh -c "$(PKG_CONFIG) --cflags $(PYTHON_PKGCONFIG_VERS) > /dev/null 2>&1 && echo y"), y)
	BUILD_PYTHON := $(PYTHON)
	BUILD_PYTHON_WORKS := 1
else
	BUILD_PYTHON := report_nopythondev
	NO_PYTHON = 1
endif
endif # NO_PYTHON

export BUILD_PYTHON_WORKS
export NO_PYTHON

# $(call test-build, snippet, ret) -> ret if snippet compiles
#                                  -> empty otherwise
test-build = $(if $(shell sh -c 'echo "$(1)" | \
	$(CC) -o /dev/null -x c - > /dev/null 2>&1 && echo y'), $2)

UDIS86_AVAILABLE := $(call test-build,\#include <udis86.h>, y)
ifneq ($(strip $(UDIS86_AVAILABLE)), y)
NO_UDIS86 := 1
endif

ifndef NO_UDIS86
# have udis86 disassembler library?
udis86-flags := -DHAVE_UDIS86 -ludis86
udis86-ldflags := -ludis86
endif # NO_UDIS86

define BLK_TC_FLUSH_SOURCE
#include <linux/blktrace_api.h>
int main(void) { return BLK_TC_FLUSH; }
endef

# have flush/fua block layer instead of barriers?
blk-flags := $(call test-build,$(BLK_TC_FLUSH_SOURCE),-DHAVE_BLK_TC_FLUSH)

define MEMFD_CREATE_SOURCE
#define _GNU_SOURCE
#include <sys/mman.h>
int main(void) { return memfd_create(\"test\", 0); }
endef

# have memfd_create available
memfd-flags := $(call test-build,$(MEMFD_CREATE_SOURCE),-DHAVE_MEMFD_CREATE)

ifeq ("$(origin O)", "command line")

  saved-output := $(O)
  BUILD_OUTPUT := $(shell cd $(O) && /bin/pwd)
  $(if $(BUILD_OUTPUT),, \
    $(error output directory "$(saved-output)" does not exist))

else
  BUILD_OUTPUT = $(CURDIR)
endif

srctree		:= $(if $(BUILD_SRC),$(BUILD_SRC),$(CURDIR))
objtree		:= $(BUILD_OUTPUT)
src		:= $(srctree)
obj		:= $(objtree)

PKG_CONFIG_SOURCE_FILE = libtracecmd.pc
PKG_CONFIG_FILE := $(addprefix $(BUILD_OUTPUT)/,$(PKG_CONFIG_SOURCE_FILE))

export pkgconfig_dir PKG_CONFIG_FILE

export prefix bindir src obj

LIBS = -ldl

LIBTRACECMD_DIR = $(obj)/lib/trace-cmd
LIBTRACECMD_STATIC = $(LIBTRACECMD_DIR)/libtracecmd.a
LIBTRACECMD_SHARED = $(LIBTRACECMD_DIR)/libtracecmd.so.$(LIBTRACECMD_VERSION)
LIBTRACECMD_SHARED_VERSION = $(shell echo $(LIBTRACECMD_SHARED) | sed -e 's/\(\.so\.[0-9]*\).*/\1/')
LIBTRACECMD_SHARED_SO = $(shell echo $(LIBTRACECMD_SHARED) | sed -e 's/\(\.so\).*/\1/')

export LIBTRACECMD_STATIC LIBTRACECMD_SHARED
export LIBTRACECMD_SHARED_VERSION LIBTRACECMD_SHARED_SO

LIBTRACEEVENT=libtraceevent
LIBTRACEFS=libtracefs

TEST_LIBTRACEEVENT = $(shell sh -c "$(PKG_CONFIG) --atleast-version $(LIBTRACEEVENT_MIN_VERSION) $(LIBTRACEEVENT) > /dev/null 2>&1 && echo y")
TEST_LIBTRACEFS = $(shell sh -c "$(PKG_CONFIG) --atleast-version $(LIBTRACEFS_MIN_VERSION) $(LIBTRACEFS) > /dev/null 2>&1 && echo y")

ifeq ("$(TEST_LIBTRACEEVENT)", "y")
LIBTRACEEVENT_CFLAGS = $(shell sh -c "$(PKG_CONFIG) --cflags $(LIBTRACEEVENT)")
LIBTRACEEVENT_LDLAGS = $(shell sh -c "$(PKG_CONFIG) --libs $(LIBTRACEEVENT)")
else
.PHONY: warning
warning:
	@echo "********************************************"
	@echo "** NOTICE: libtraceevent version $(LIBTRACEEVENT_MIN_VERSION) or higher not found on system"
	@echo "**"
	@echo "** Consider installing the latest libtraceevent from your"
	@echo "** distribution, or from source:"
	@echo "**"
	@echo "**  https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git/ "
	@echo "**"
	@echo "********************************************"
endif

export LIBTRACEEVENT_CFLAGS LIBTRACEEVENT_LDLAGS

ifeq ("$(TEST_LIBTRACEFS)", "y")
LIBTRACEFS_CFLAGS = $(shell sh -c "$(PKG_CONFIG) --cflags $(LIBTRACEFS)")
LIBTRACEFS_LDLAGS = $(shell sh -c "$(PKG_CONFIG) --libs $(LIBTRACEFS)")
else
.PHONY: warning
warning:
	@echo "********************************************"
	@echo "** NOTICE: libtracefs version $(LIBTRACEFS_MIN_VERSION) or higher not found on system"
	@echo "**"
	@echo "** Consider installing the latest libtracefs from your"
	@echo "** distribution, or from source:"
	@echo "**"
	@echo "**  https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/ "
	@echo "**"
	@echo "********************************************"
endif

export LIBTRACEFS_CFLAGS LIBTRACEFS_LDLAGS

TRACE_LIBS = -L$(LIBTRACECMD_DIR) -ltracecmd	\
	     $(LIBTRACEEVENT_LDLAGS) $(LIBTRACEFS_LDLAGS)

export LIBS TRACE_LIBS
export LIBTRACECMD_DIR
export Q SILENT VERBOSE EXT

# Include the utils
include scripts/utils.mk

INCLUDES = -I$(src)/include -I$(src)/../../include
INCLUDES += -I$(src)/include/trace-cmd
INCLUDES += -I$(src)/lib/trace-cmd/include
INCLUDES += -I$(src)/lib/trace-cmd/include/private
INCLUDES += -I$(src)/tracecmd/include
INCLUDES += $(LIBTRACEEVENT_CFLAGS)
INCLUDES += $(LIBTRACEFS_CFLAGS)

include $(src)/features.mk

# Set compile option CFLAGS if not set elsewhere
CFLAGS ?= -g -Wall
CPPFLAGS ?=
LDFLAGS ?=

ifndef NO_VSOCK
VSOCK_DEFINED := $(shell if (echo "$(pound)include <linux/vm_sockets.h>" | $(CC) -E - >/dev/null 2>&1) ; then echo 1; else echo 0 ; fi)
else
VSOCK_DEFINED := 0
endif

export VSOCK_DEFINED
ifeq ($(VSOCK_DEFINED), 1)
CFLAGS += -DVSOCK
endif

PERF_DEFINED := $(shell if (echo "$(pound)include <linux/perf_event.h>" | $(CC) -E - >/dev/null 2>&1) ; then echo 1; else echo 0 ; fi)
export PERF_DEFINED
ifeq ($(PERF_DEFINED), 1)
CFLAGS += -DPERF
endif

ZLIB_INSTALLED := $(shell if (printf "$(pound)include <zlib.h>\n void main(){deflateInit(NULL, Z_BEST_COMPRESSION);}" | $(CC) -o /dev/null -x c - -lz >/dev/null 2>&1) ; then echo 1; else echo 0 ; fi)
ifeq ($(ZLIB_INSTALLED), 1)
export ZLIB_INSTALLED
CFLAGS += -DHAVE_ZLIB
$(info    Have zlib compression support)
endif

TEST_LIBZSTD = $(shell sh -c "$(PKG_CONFIG) --atleast-version 1.4.0 libzstd > /dev/null 2>&1 && echo y")

ifeq ("$(TEST_LIBZSTD)", "y")
LIBZSTD_CFLAGS = $(shell sh -c "$(PKG_CONFIG) --cflags libzstd")
LIBZSTD_LDLAGS = $(shell sh -c "$(PKG_CONFIG) --libs libzstd")
CFLAGS += -DHAVE_ZSTD
ZSTD_INSTALLED=1
$(info    Have ZSTD compression support)
else
$(info	  *************************************************************)
$(info	  ZSTD package not found, best compression algorithm not in use)
$(info	  *************************************************************)
endif

export LIBZSTD_CFLAGS LIBZSTD_LDLAGS ZSTD_INSTALLED

CUNIT_INSTALLED := $(shell if (printf "$(pound)include <CUnit/Basic.h>\n void main(){CU_initialize_registry();}" | $(CC) -o /dev/null -x c - -lcunit >/dev/null 2>&1) ; then echo 1; else echo 0 ; fi)
export CUNIT_INSTALLED

export CFLAGS
export INCLUDES

# Required CFLAGS
override CFLAGS += -D_GNU_SOURCE

ifndef NO_PTRACE
ifneq ($(call try-cc,$(SOURCE_PTRACE),),y)
	NO_PTRACE = 1
	override CFLAGS += -DWARN_NO_PTRACE
endif
endif

ifdef NO_PTRACE
override CFLAGS += -DNO_PTRACE
endif

ifndef NO_AUDIT
ifneq ($(call try-cc,$(SOURCE_AUDIT),-laudit),y)
	NO_AUDIT = 1
	override CFLAGS += -DWARN_NO_AUDIT
endif
endif

ifdef NO_AUDIT
override CFLAGS += -DNO_AUDIT
else
LIBS += -laudit
endif

# Append required CFLAGS
override CFLAGS += $(INCLUDES) $(VAR_DIR)
override CFLAGS += $(PLUGIN_DIR_TRACECMD_SQ)
override CFLAGS += $(udis86-flags) $(blk-flags) $(memfd-flags)
override LDFLAGS += $(udis86-ldflags)

CMD_TARGETS = trace-cmd $(BUILD_PYTHON)

###
#    Default we just build trace-cmd
#
#    If you want all libraries, then do: make libs
###

all: all_cmd plugins show_other_make

all_cmd: $(CMD_TARGETS)

BUILD_PREFIX := $(BUILD_OUTPUT)/build_prefix

$(BUILD_PREFIX): force
	$(Q)$(call build_prefix,$(prefix))

$(PKG_CONFIG_FILE) : ${PKG_CONFIG_SOURCE_FILE}.template $(BUILD_PREFIX) $(VERSION_FILE)
	$(Q) $(call do_make_pkgconfig_file,$(prefix))

trace-cmd: force $(LIBTRACECMD_STATIC) \
	force $(obj)/lib/trace-cmd/plugins/tracecmd_plugin_dir
	$(Q)$(MAKE) -C $(src)/tracecmd $(obj)/tracecmd/$@

$(LIBTRACECMD_STATIC): force
	$(Q)$(MAKE) -C $(src)/lib/trace-cmd $@

$(LIBTRACECMD_SHARED): force
	$(Q)$(MAKE) -C $(src)/lib/trace-cmd libtracecmd.so

libtracecmd.a: $(LIBTRACECMD_STATIC)
libtracecmd.so: $(LIBTRACECMD_SHARED)

libs: $(LIBTRACECMD_SHARED) $(PKG_CONFIG_FILE)

VERSION = $(LIBTC_VERSION)
PATCHLEVEL = $(LIBTC_PATCHLEVEL)
EXTRAVERSION = $(LIBTC_EXTRAVERSION)

define make_version.h
  (echo '/* This file is automatically generated. Do not modify. */';		\
   echo \#define VERSION_CODE $(shell						\
   expr $(VERSION) \* 256 + $(PATCHLEVEL));					\
   echo '#define EXTRAVERSION ' $(EXTRAVERSION);				\
   echo '#define VERSION_STRING "'$(VERSION).$(PATCHLEVEL).$(EXTRAVERSION)'"';	\
  ) > $1
endef

define update_version.h
  ($(call make_version.h, $@.tmp);		\
    if [ -r $@ ] && cmp -s $@ $@.tmp; then	\
      rm -f $@.tmp;				\
    else					\
      echo '  UPDATE                 $@';	\
      mv -f $@.tmp $@;				\
    fi);
endef

$(VERSION_FILE): force
	$(Q)$(call update_version.h)

gui: force
	@echo "***************************"
	@echo "  KernelShark has moved!"
	@echo "  Please use its new home at https://git.kernel.org/pub/scm/utils/trace-cmd/kernel-shark.git/"
	@echo "***************************"

test: force trace-cmd
ifneq ($(CUNIT_INSTALLED),1)
	$(error CUnit framework not installed, cannot build unit tests))
endif
	$(Q)$(MAKE) -C $(src)/utest $@

plugins_tracecmd: force $(obj)/lib/trace-cmd/plugins/tracecmd_plugin_dir
	$(Q)$(MAKE) -C $(src)/lib/trace-cmd/plugins

plugins: plugins_tracecmd

$(obj)/lib/trace-cmd/plugins/tracecmd_plugin_dir: force
	$(Q)$(MAKE) -C $(src)/lib/trace-cmd/plugins $@

show_other_make:
	@echo "Note: to build man pages, type \"make doc\""
	@echo "      to build unit tests, type \"make test\""

PHONY += show_other_make

define find_tag_files
	find . -name '\.pc' -prune -o -name '*\.[ch]' -print -o -name '*\.[ch]pp' \
		! -name '\.#' -print
endef

tags:	force
	$(RM) tags
	$(call find_tag_files) | xargs ctags --extra=+f --c-kinds=+px

TAGS:	force
	$(RM) TAGS
	$(call find_tag_files) | xargs etags

cscope: force
	$(RM) cscope*
	$(call find_tag_files) | cscope -b -q

install_plugins_tracecmd: force
	$(Q)$(MAKE) -C $(src)/lib/trace-cmd/plugins install_plugins

install_plugins: install_plugins_tracecmd

install_python: force
	$(Q)$(MAKE) -C $(src)/python $@

install_bash_completion: force
	$(Q)$(call do_install_data,$(src)/tracecmd/trace-cmd.bash,$(BASH_COMPLETE_DIR))

install_cmd: all_cmd install_plugins install_python install_bash_completion
	$(Q)$(call do_install,$(obj)/tracecmd/trace-cmd,$(bindir_SQ))

install: install_cmd
	@echo "Note: to install man pages, type \"make install_doc\""

install_gui: force
	@echo "Nothing to do here."
	@echo " Have you tried https://git.kernel.org/pub/scm/utils/trace-cmd/kernel-shark.git/"

install_libs: libs
	$(Q)$(MAKE) -C $(src)/lib/trace-cmd/ $@

doc:
	$(MAKE) -C $(src)/Documentation all

doc_clean:
	$(MAKE) -C $(src)/Documentation clean

install_doc:
	$(MAKE) -C $(src)/Documentation install

clean:
	$(RM) *.o *~ *.a *.so .*.d
	$(RM) tags TAGS cscope* $(PKG_CONFIG_SOURCE_FILE) $(VERSION_FILE)
	$(MAKE) -C $(src)/lib/trace-cmd clean
	$(MAKE) -C $(src)/lib/trace-cmd/plugins clean
	$(MAKE) -C $(src)/utest clean
	$(MAKE) -C $(src)/python clean
	$(MAKE) -C $(src)/tracecmd clean

define build_uninstall_script
	$(Q)mkdir $(BUILD_OUTPUT)/tmp_build
	$(Q)$(MAKE) -C $(src) DESTDIR=$(BUILD_OUTPUT)/tmp_build O=$(BUILD_OUTPUT) $1 > /dev/null
	$(Q)find $(BUILD_OUTPUT)/tmp_build ! -type d -printf "%P\n" > $(BUILD_OUTPUT)/build_$2
	$(Q)$(RM) -rf $(BUILD_OUTPUT)/tmp_build
endef

build_uninstall: $(BUILD_PREFIX)
	$(call build_uninstall_script,install,uninstall)

$(BUILD_OUTPUT)/build_uninstall: build_uninstall

build_libs_uninstall: $(BUILD_PREFIX)
	$(call build_uninstall_script,install_libs,libs_uninstall)

$(BUILD_OUTPUT)/build_libs_uninstall: build_libs_uninstall

define uninstall_file
	if [ -f $(DESTDIR)/$1 -o -h $(DESTDIR)/$1 ]; then \
		$(call print_uninstall,$(DESTDIR)/$1)$(RM) $(DESTDIR)/$1; \
	fi;
endef

uninstall: $(BUILD_OUTPUT)/build_uninstall
	@$(foreach file,$(shell cat $(BUILD_OUTPUT)/build_uninstall),$(call uninstall_file,$(file)))

uninstall_libs: $(BUILD_OUTPUT)/build_libs_uninstall
	@$(foreach file,$(shell cat $(BUILD_OUTPUT)/build_libs_uninstall),$(call uninstall_file,$(file)))

##### PYTHON STUFF #####

report_noswig: force
	$(Q)echo
	$(Q)echo "    NO_PYTHON forced: swig not installed, not compiling python plugins"
	$(Q)echo

report_nopythondev: force
	$(Q)echo
	$(Q)echo "    python-dev is not installed, not compiling python plugins"
	$(Q)echo

ifndef NO_PYTHON
PYTHON_INCLUDES = `$(PKG_CONFIG) --cflags $(PYTHON_PKGCONFIG_VERS)`
PYTHON_LDFLAGS = `$(PKG_CONFIG) --libs $(PYTHON_PKGCONFIG_VERS)` \
		$(shell $(PYTHON_VERS)-config --ldflags)
PYGTK_CFLAGS = `$(PKG_CONFIG) --cflags pygtk-2.0`
else
PYTHON_INCLUDES =
PYTHON_LDFLAGS =
PYGTK_CFLAGS =
endif

export PYTHON_INCLUDES
export PYTHON_LDFLAGS
export PYGTK_CFLAGS

ctracecmd.so: force $(LIBTRACECMD_STATIC)
	$(Q)$(MAKE) -C $(src)/python $@

PHONY += python
python: $(PYTHON)


dist:
	git archive --format=tar --prefix=trace-cmd-$(TRACECMD_VERSION)/ HEAD \
		> ../trace-cmd-$(TRACECMD_VERSION).tar
	cat ../trace-cmd-$(TRACECMD_VERSION).tar | \
		bzip2 -c9 > ../trace-cmd-$(TRACECMD_VERSION).tar.bz2
	cat ../trace-cmd-$(TRACECMD_VERSION).tar | \
		xz -e -c8 > ../trace-cmd-$(TRACECMD_VERSION).tar.xz

PHONY += force
force:

# Declare the contents of the .PHONY variable as phony.  We keep that
# information in a variable so we can use it in if_changed and friends.
.PHONY: $(PHONY)