mesa: Disable use of "pragma export" on OS X. clang doesn't understand it.

include/GL/gl.h

 /*
  * Mesa 3-D graphics library
  * Version:  7.6
  *
  * Copyright (C) 1999-2006  Brian Paul   All Rights Reserved.
  * Copyright (C) 2009  VMware, Inc.  All Rights Reserved.
  *
  * Permission is hereby granted, free of charge, to any person obtaining a
  * copy of this software and associated documentation files (the "Software"),
  * to deal in the Software without restriction, including without limitation
@@ skipping 54 matching lines @@
 #    define GLAPIENTRY __stdcall
 #  endif
 #elif defined(__CYGWIN__) && defined(USE_OPENGL32) /* use native windows opengl32 */
 #  define GLAPI extern
 #  define GLAPIENTRY __stdcall
 #elif (defined(__GNUC__) && __GNUC__ >= 4) || (defined(__SUNPRO_C) && (__SUNPRO_C >= 0x590))
 #  define GLAPI __attribute__((visibility("default")))
 #  define GLAPIENTRY
 #endif /* WIN32 && !CYGWIN */
 
-#if (defined(__BEOS__) && defined(__POWERPC__)) || defined(__QUICKDRAW__)
+/* See https://bugs.freedesktop.org/show_bug.cgi?id=77749 */
+#if (defined(__BEOS__) && defined(__POWERPC__)) /* || defined(__QUICKDRAW__) */
 #  define PRAGMA_EXPORT_SUPPORTED               1
 #endif
 
 /*
  * WINDOWS: Include windows.h here to define APIENTRY.
  * It is also useful when applications include this file by
  * including only glut.h, since glut.h depends on windows.h.
  * Applications needing to include windows.h with parms other
  * than "WIN32_LEAN_AND_MEAN" may include windows.h before
  * glut.h or gl.h.
@@ skipping 2140 matching lines @@
 /*
  * End system-specific stuff
  **********************************************************************/
 
 
 #ifdef __cplusplus
 }
 #endif
 
 #endif /* __gl_h_ */