gl_common: fix compilation on OS X 10.8

As of OS X 10.8 Apple completely removed X11 from the system.
gl_common.h was including gl.h using the path <GL/gl.h>. This path
comes from the X11 headers, which are missing in 10.8.

Change gl_common.h to include gl.h from Apple's OpenGL implementation
as <OpenGL/gl.h> if X11/XQuartz is not detected.

libvo/gl_common.h

@@ -33,8 +33,14 @@
 #include "video_out.h"
 #include "csputils.h"
 
+#if defined(CONFIG_GL_COCOA) && !defined(CONFIG_GL_X11)
+#include <OpenGL/gl.h>
+#include <OpenGL/gl3.h>
+#include <OpenGL/glext.h>
+#else
 #include <GL/gl.h>
 #include <GL/glext.h>
+#endif
 
 #include "libvo/gl_header_fixes.h"