sdl: workaround bug in sdl 2.0.8 headers

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=892087Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Message-id: 20180307154258.9313-1-kraxel@redhat.com
(cherry picked from commit 2ca5c430)
Signed-off-by: NGreg Kurz <groug@kaod.org>

configure

@@ -2788,6 +2788,7 @@ if test "$sdl" != "no" ; then
 int main( void ) { return SDL_Init (SDL_INIT_VIDEO); }
 EOF
   sdl_cflags=$($sdlconfig --cflags 2>/dev/null)
+  sdl_cflags="$sdl_cflags -Wno-undef"  # workaround 2.0.8 bug
   if test "$static" = "yes" ; then
     if $pkg_config $sdlname --exists; then
       sdl_libs=$($pkg_config $sdlname --static --libs 2>/dev/null)