Don't init the Big Clutter Lock on Win32

Since the Windows GUI system is assuming multithreadedness, initializing
locks after entering the GUI portion on Windows is likely to cause
problems[1][2], which results many Clutter programs to crash due to
releasing resources that they did not own.

[1]: Multi-threaded use of GTK+ on Win32 in README.win32 of GTK+ source package
[2]: Explanation of Windows GUI system regarding its multithreadness
     assumtopns-http://lists-archives.org/gtk/12724-compiling-a-gtk-application-on-windows.html

clutter/clutter-main.c

@@ -215,11 +215,13 @@ clutter_threads_init_default (void)
 {
   g_mutex_init (&clutter_threads_mutex);
 
+#ifndef HAVE_CLUTTER_WIN32
   if (clutter_threads_lock == NULL)
     clutter_threads_lock = clutter_threads_impl_lock;
 
   if (clutter_threads_unlock == NULL)
     clutter_threads_unlock = clutter_threads_impl_unlock;
+#endif
 }
 
 #define ENVIRONMENT_GROUP       "Environment"