Check whether rstcheck and mdl are installed, and give hint on how to…

… install them if they are not available.

Signed-off-by: Felix Fontein <felix@fontein.de>

Makefile

@@ -20,6 +20,9 @@ SYFT_VERSION        ?= v0.87.0
 GORELEASER          := $(BIN_DIR)/goreleaser
 GORELEASER_VERSION  ?= v1.20.0
 
+RSTCHECK            := $(shell command -v rstcheck)
+MARKDOWNLINT        := $(shell command -v mdl)
+
 export PATH := $(BIN_DIR):$(PATH)
 
 .PHONY: all
@@ -52,12 +55,14 @@ checkdocs: checkrst checkmd
 .PHONY: checkrst
 RST_FILES=$(shell find . -name '*.rst' | grep -v /vendor/ | sort)
 checkrst: $(RST_FILES)
-	rstcheck --report-level warning $^
+	@if [ "$(RSTCHECK)" == "" ]; then echo "Need rstcheck to lint RST files. Install rstcheck from your system package repository or from PyPI (https://pypi.org/project/rstcheck/)."; exit 1; fi
+	$(RSTCHECK) --report-level warning $^
 
 .PHONY: checkmd
 MD_FILES=$(shell find . -name '*.md' | grep -v /vendor/ | sort)
 checkmd: $(MD_FILES)
-	mdl $^
+	@if [ "$(MARKDOWNLINT)" == "" ]; then echo "Need markdownlint to lint RST files. Install markdownlint from your system package repository or from https://github.com/markdownlint/markdownlint."; exit 1; fi
+	$(MARKDOWNLINT) $^
 
 .PHONY: test
 test: vendor