Race condition when QThread creates a QEventDispatcherGlib instance

There's a race condition when QThread creates a QEventDispatcherGlib instance, the following code can be found in the QEventDispatcherGlibPrivate() c'tor

if (!g_thread_supported())
g_thread_init(NULL);

This is can cause g_thread_init() to be called twice when creating two threads at the same time and the main event loop created by QApplication is not a glib one (i.e. when using QtMotif). This causes g_thread_init() to abort.