* acinclude.m4: add LIBVIRT_COMPILE_WARNINGS, copied from
GNOME but with a few more flags we'd been using.
* configure.in: use that instead of setting CFLAGS
directly.
* proxy/Makefile.am, python/Makefile.am, qemud/Makefile.am,
src/Makefile.am, tests/Makefile.am: use $(WARN_CFLAGS)