Fix macOS compilation (#384)

Disable X11 on macOS, I don't know why it's enabled

src/Nazara/Platform/SDL2/WindowImpl.cpp

@@ -13,6 +13,13 @@
 #include <Nazara/Platform/SDL2/SDLHelper.hpp>
 #include <Nazara/Utility/Image.hpp>
 #include <SDL.h>
+
+#ifdef NAZARA_PLATFORM_MACOS
+// I'm not sure why, but SDL_VIDEO_DRIVER_X11 is automatically defined here by SDL_config.h
+// This is problematic as it requires X11/X.h which is not present (adding libxext/libx11/xorgproto packages didn't help)
+#undef SDL_VIDEO_DRIVER_X11
+#endif
+
 #include <SDL_syswm.h>
 #include <Utfcpp/utf8.h>
 #include <cstdio>