build: silence some compiler warnings from gnulib

build: silence some compiler warnings from gnulib

Gnulib claims that there are some classes of warnings that are
worth enabling during development, but where silencing those
warnings causes code bloat that is not necessary in an optimized
build.  The code bloat to silence the warnings is only enabled
by -Dlint.  Follow the lead of coreutils in setting up -Dlint
whenever full warnings are requested.

* m4/virt-compile-warnings.m4 (LIBVIRT_COMPILE_WARNINGS): Add
-Dlint, and move _FORTIFY_SOURCE to config.h instead of CFLAGS.