* 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)