Fixed a few issues compiling with Mac OS X 10.6

Unfortunately the audio and cdrom systems do not build at all, and if you
disable those, SDL still fails to link with these errors:

Undefined symbols:
  "_OBJC_IVAR_$_NSScreen._frame", referenced from:
      -[NSScreen(NSScreenAccess) setFrame:] in SDL_cocoamodes.o
  "_KLGetKeyboardLayoutProperty", referenced from:
      _UpdateKeymap in SDL_cocoakeyboard.o
      _UpdateKeymap in SDL_cocoakeyboard.o
  "_KLGetCurrentKeyboardLayout", referenced from:
      _UpdateKeymap in SDL_cocoakeyboard.o
ld: symbol(s) not found

include/SDL_opengl.h

@@ -22,6 +22,9 @@
 
 /* This is a simple file to encapsulate the OpenGL API headers */
 
+#ifndef _SDL_opengl_h
+#define _SDL_opengl_h
+
 #include "SDL_config.h"
 
 #ifdef __WIN32__
@@ -37,6 +40,7 @@
 #if defined(__MACOSX__)
 #include <OpenGL/gl.h>          /* Header File For The OpenGL Library */
 #include <OpenGL/glu.h>         /* Header File For The GLU Library */
+#define __X_GL_H
 #else
 #include <GL/gl.h>              /* Header File For The OpenGL Library */
 #include <GL/glu.h>             /* Header File For The GLU Library */
@@ -9307,3 +9311,7 @@ typedef void (APIENTRYP PFNGLGETOBJECTPARAMETERIVAPPLEPROC) (GLenum objectType,
 #endif
 /* *INDENT-ON* */
 #endif /* NO_SDL_GLEXT */
+
+#endif /* _SDL_opengl_h */
+
+/* vi: set ts=4 sw=4 expandtab: */

src/video/x11/SDL_x11events.c

@@ -25,11 +25,12 @@
 #include <sys/time.h>
 #include <unistd.h>
 
-#include "SDL_syswm.h"
 #include "SDL_x11video.h"
 #include "../../events/SDL_events_c.h"
 #include "../../events/SDL_mouse_c.h"
 
+#include "SDL_syswm.h"
+
 static void
 X11_DispatchEvent(_THIS)
 {

src/video/x11/SDL_x11opengl.c

@@ -39,6 +39,10 @@
 #define DEFAULT_OPENGL	"libGL.so.1"
 #endif
 
+#ifndef GLX_NONE_EXT
+#define GLX_NONE_EXT                       0x8000
+#endif
+
 #ifndef GLX_ARB_multisample
 #define GLX_ARB_multisample
 #define GLX_SAMPLE_BUFFERS_ARB             100000

src/video/x11/SDL_x11window.c

@@ -21,7 +21,6 @@
 */
 #include "SDL_config.h"
 
-#include "SDL_syswm.h"
 #include "../SDL_sysvideo.h"
 #include "../../events/SDL_keyboard_c.h"
 #include "../../events/SDL_mouse_c.h"
@@ -35,6 +34,8 @@
 #include "SDL_x11opengles.h"
 #endif
 
+#include "SDL_syswm.h"
+
 #define _NET_WM_STATE_REMOVE    0l
 #define _NET_WM_STATE_ADD       1l
 #define _NET_WM_STATE_TOGGLE    2l