makefiles/cflags.inc.mk: change optional CFLAGS to 'OPTIONAL_CFLAGS'

Update the optional flags to use 'OPTIONAL_CFLAGS' instead of evaluating
everytime.

They are now blacklisted by architecture/toolchain according to the
current docker reference image.

makefiles/cflags.inc.mk

@@ -11,37 +11,19 @@ endif
 # 0x0 might be a sane memory location for embedded systems, so the test must not be removed.
 # Right now clang does not use the *delete-null-pointer* optimization, and does not understand the parameter.
 # Related issues: #628, #664.
-ifeq ($(shell $(CC) -fno-delete-null-pointer-checks -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
-  ifeq ($(shell LANG=C $(CC) -fno-delete-null-pointer-checks -E - 2>&1 1>/dev/null </dev/null | grep warning: | grep -- -fno-delete-null-pointer-checks),)
-    CFLAGS += -fno-delete-null-pointer-checks
+OPTIONAL_CFLAGS += -fno-delete-null-pointer-checks
 
-    ifneq ($(shell $(CXX) -fno-delete-null-pointer-checks -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
-      CXXUWFLAGS += -fno-delete-null-pointer-checks
-    else
-      ifneq ($(shell LANG=C $(CXX) -fno-delete-null-pointer-checks -E - 2>&1 1>/dev/null </dev/null | grep warning: | grep -- -fno-delete-null-pointer-checks),)
-        CXXUWFLAGS += -fno-delete-null-pointer-checks
-      endif
-    endif
-  endif
-endif
-
-# Template for testing a compiler flag and adding it to CFLAGS (errors usually
-# happens when using older toolchains which do not support the given flags)
-define cflags_test_and_add
-  ifeq ($(shell $(CC) -Werror $(1) -E - 2>/dev/null >/dev/null </dev/null ; echo $$?),0)
-    CFLAGS += $(1)
-  endif
-endef
 # Use colored compiler output if the compiler supports this and if this is not
 # disabled by the user
 ifeq ($(CC_NOCOLOR),0)
-  $(eval $(call cflags_test_and_add,-fdiagnostics-color))
+  OPTIONAL_CFLAGS += -fdiagnostics-color
 endif
 
 # Fast-out on old style function definitions.
 # They cause unreadable error compiler errors on missing semicolons.
 # Worse yet they hide errors by accepting wildcard argument types.
-$(foreach flag,-Wstrict-prototypes -Wold-style-definition,$(eval $(call cflags_test_and_add,$(flag))))
+OPTIONAL_CFLAGS += -Wstrict-prototypes
+OPTIONAL_CFLAGS += -Wold-style-definition
 
 # Unwanted flags for c++
 CXXUWFLAGS += -std=%
@@ -59,7 +41,9 @@ CFLAGS += -fno-common
 # Enable all default warnings and all extra warnings
 CFLAGS += -Wall -Wextra
 # Enable additional checks for printf/scanf format strings
-$(foreach flag,-Wformat=2 -Wformat-overflow -Wformat-truncation,$(eval $(call cflags_test_and_add,$(flag))))
+OPTIONAL_CFLAGS += -Wformat=2
+OPTIONAL_CFLAGS += -Wformat-overflow
+OPTIONAL_CFLAGS += -Wformat-truncation
 
 # Warn if a user-supplied include directory does not exist.
 CFLAGS += -Wmissing-include-dirs