add a minimal config and makefile to test watcom/os2 builds.

include/SDL_config.h

@@ -41,8 +41,10 @@
 #include "SDL_config_android.h"
 #elif defined(__PSP__)
 #include "SDL_config_psp.h"
+#elif defined(__OS2__)
+#include "SDL_config_os2.h"
 #else
-/* This is a minimal configuration just to get SDL running on new platforms */
+/* This is a minimal configuration just to get SDL running on new platforms. */
 #include "SDL_config_minimal.h"
 #endif /* platform config */