opengl: make SDL_opengl_glext.h's include compatible with macos

- Mesa defines __gl_glext_h_
- Apple defines __glext_h_

include/SDL_opengl_glext.h

@@ -1,4 +1,5 @@
-#ifndef __gl_glext_h_
+#if !defined(__glext_h_) && !defined(__gl_glext_h_)
+#define __glext_h_ 1
 #define __gl_glext_h_ 1
 
 #ifdef __cplusplus