Cleanup config.mk, introduce $CPPFLAGS
Cleanup config.mk, introduce $CPPFLAGS

$CPPFLAGS does only contain feature test macro defines, thus one
system which need different ones compilation should succeed with
something like:

 make CPPFLAGS= LIBS=-lc

phillid/abduco,phillid/abduco,martanne/abduco,xinhaoyuan/abduco-mk,xinhaoyuan/abduco-mk,martanne/abduco