Fixed bug 2568 - NO_SDL_GLEXT should prevent OS glext.h as well

J?rgen Tjern?

If you #define NO_SDL_GLEXT before including SDL_opengl.h, it still includes the platform-provided glext.h. The comments indicate that this define is intended to be used when you provide your own glext.h (quote from SDL_opengl.h: "Define this if you have your own version of glext.h and want to disable the version included in SDL_opengl.h.")

This is a problem because glext.h depends on the contents of gl.h, and it's practical to let SDL_opengl.h pick the right #include for gl.h for our platform.

include/SDL_opengl.h

@@ -45,8 +45,8 @@
 #ifdef __glext_h_
 /* Someone has already included glext.h */
 #define NO_SDL_GLEXT
-#endif
-#ifndef NO_SDL_GLEXT
+#else
+#define _SDL_CLEAR_GLEXT_HEADERGUARD
 #define __glext_h_              /* Don't let gl.h include glext.h */
 #endif
 #if defined(__MACOSX__)
@@ -55,7 +55,7 @@
 #else
 #include <GL/gl.h>              /* Header File For The OpenGL Library */
 #endif
-#ifndef NO_SDL_GLEXT
+#ifdef _SDL_CLEAR_GLEXT_HEADERGUARD
 #undef __glext_h_
 #endif