maint: allow per-directory CFLAGS settings with non-recursive make

maint: allow per-directory CFLAGS settings with non-recursive make

* src/local.mk (AM_CFLAGS): Don't use $(WARN_CFLAGS) here.
* cfg.mk (src_CFLAGS, lib_CFLAGS, gnulib-tests_CFLAGS): Define here
instead.
(AM_CFLAGS): Augment using the above.
* configure.ac: Note that the configure-time option,
--enable-gcc-warnings now functions only when using GNU make.
Well, currently it does still work in gnulib-tests, but that should
soon be fixed.
Improved-by: Stefano Lattarini