SDL: Fix build with SDL 2.0.14 after KMOD_GUI change to enum

Since SDL 2.0.14, KMOD_GUI is no longer a macro but an enum value:
https://hg.libsdl.org/SDL/rev/15a0bc9612e9

So this code broke as the enum value doesn't satisfy `defined()`.

src/sdl/SDL.cpp

@@ -878,7 +878,8 @@ void sdlInitVideo()
 
     sdlResizeVideo();
 }
-#if defined(KMOD_GUI)
+
+#ifndef KMOD_META
 #define KMOD_META KMOD_GUI
 #endif