win32: Allow POSIX threads to be used if --with-threads=posix

All tests pass with this patch AND a good pthreads implementation
(i'm using winpthreads, not pthreads-w32).

https://bugzilla.gnome.org/show_bug.cgi?id=697626

glib/glib-init.c

@@ -239,12 +239,16 @@ DllMain (HINSTANCE hinstDLL,
     case DLL_PROCESS_ATTACH:
       glib_dll = hinstDLL;
       g_clock_win32_init ();
+#ifdef THREADS_WIN32
       g_thread_win32_init ();
+#endif
       glib_init ();
       break;
 
     case DLL_THREAD_DETACH:
+#ifdef THREADS_WIN32
       g_thread_win32_thread_detach ();
+#endif
       break;
 
     default: