configure: fix OpenGL autodetection on OS X

Was broken from 746b5e6 since the OpenGL headers are under OpenGL/ and not GL/
on OS X. Thanks to @Kovensky for the initial patch.

configure

@@ -2177,14 +2177,23 @@ EOF
   fi
 
   cat > $TMPC << EOF
+#ifdef __APPLE__
+#include <OpenGL/gl.h>
+#include <OpenGL/glext.h>
+#else
 #include <GL/gl.h>
 #include <GL/glext.h>
+#endif
 int main(int argc, char *argv[]) {
   return !GL_INVALID_FRAMEBUFFER_OPERATION;
 }
 EOF
   if ! cc_check; then
     _gl=no
+    _gl_x11=no
+    _gl_wayland=no
+    _gl_win32=no
+    _gl_cocoa=no
     res_comment="missing glext.h, get from http://www.opengl.org/registry/api/glext.h"
   fi
 else