Fixes build with GLX on Mac OS X.

Moves preprocessor #ifdef __linux_ above else statement, avoiding the
lack of an else block if __linux__ is not defined.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2212

Signed-off-by: Emmanuele Bassi <ebassi@linux.intel.com>

clutter/glx/clutter-stage-glx.c

@@ -476,8 +476,8 @@ wait_for_vblank (ClutterBackendGLX *backend_glx)
                                     (retraceCount + 1) % 2,
                                     &retraceCount);
     }
-  else
 #ifdef __linux__
+  else
     {
       drm_wait_vblank_t blank;