This way, overriding CFLAGS on make command line keeps glib-cflags
and doesn't break the build.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20160925205748.6280-2-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>