GNU Make: force recompile when flags change

Makefile

# Make OBJS depend on CPP/C/CXX/LD flags 
$(OBJS): $(BUILDDIR)/flags.txt

# Creates a new flags.txt file when flags change
# make will detect this change and force a rebuild of
# all the sources
.PHONY: flags
$(BUILDDIR)/flags.txt: flags
	$(Q){                                  \
	TMP=`mktemp`;                          \
	echo 'CPPFLAGS: $(CPPFLAGS)' >> $$TMP; \
	echo 'CFLAGS:   $(CFLAGS)'   >> $$TMP; \
	echo 'CXXFLAGS: $(CXXFLAGS)' >> $$TMP; \
	echo 'LDFLAGS:  $(LDFLAGS)'  >> $$TMP; \
	cmp -s $$TMP $@ || mv -f $$TMP $@;     \
	}