message: Exit with an error if re2c is not available
  Instead of overwriting src/json.hpp with an empty file
diff: .PHONY: pretty clean ChangeLog.md
  # used programs
  -RE2C = re2c
  +RE2C := $(shell command -v re2c 2> /dev/null)
  SED = sed
  # main target
  @@ -185,6 +185,9 @@ clang_sanitize: clean
  # create scanner with re2c
  re2c: src/json.hpp.re2c
  +ifndef RE2C
  + $(error "re2c is not available, please install re2c")
  +endif
  $(RE2C) -W --utf-8 --encoding-policy fail --bit-vectors --nested-ifs --no-debug-info $< | $(SED) '1d' > src/json.hpp
  # pretty printer
  