opengl32: fix a compiler warning on Mac OS X

[Austin English]

GL_GLEXT_PROTOTYPES is defined in GL/gl.h on Mac. Other #ifdef methods
end up breaking compilations elsewhere.

-- 
-Austin
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diff --git a/dlls/opengl32/opengl_ext.h b/dlls/opengl32/opengl_ext.h
index 8ae7c2f..fd107c4 100644
--- a/dlls/opengl32/opengl_ext.h
+++ b/dlls/opengl32/opengl_ext.h
@@ -24,7 +24,9 @@
 #undef WINAPI
 
 #define XMD_H /* This is to prevent the Xmd.h inclusion bug :-/ */
+#ifndef __APPLE__ /* Apple's X11 define's GL_GLEXT_PROTOTYPES in GL/gl.h, causing a conflict */
 #define GL_GLEXT_PROTOTYPES
+#endif
 #include <GL/gl.h>
 #ifdef HAVE_GL_GLEXT_H
 # include <GL/glext.h>