ui/sdl: build as module

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20180301100547.18962-14-kraxel@redhat.com
diff --git a/configure b/configure
index 269add1..cb4fdb3 100755
--- a/configure
+++ b/configure
@@ -6039,7 +6039,7 @@
   echo "X11_LIBS=$x11_libs" >> $config_host_mak
 fi
 if test "$sdl" = "yes" ; then
-  echo "CONFIG_SDL=y" >> $config_host_mak
+  echo "CONFIG_SDL=m" >> $config_host_mak
   echo "CONFIG_SDLABI=$sdlabi" >> $config_host_mak
   echo "SDL_CFLAGS=$sdl_cflags" >> $config_host_mak
   echo "SDL_LIBS=$sdl_libs" >> $config_host_mak