[Makefile] How to show debug message

$(warning happy)
Show debug message

$(warning /sort$(if $(COMSPEC),.exe))
if $(COMSPEC) is true  --> print .exe
if $(COMSPEC) is wrong --> no print .exe

$(warning $(subst /,\,$(RULES_DIR)))
if $(RULES_DIR) is true  --> use / to replace \

Reference:

• Debugging Makefiles