Add a warning to g_thread_init_with_errorcheck_mutexes

Should anybody try to use this, we should at least tell them that
its gone.

gthread/gthread-impl.c

@@ -44,4 +44,5 @@ void
 g_thread_init_with_errorcheck_mutexes (gpointer vtable)
 {
   g_assert (vtable == NULL);
+  g_warning ("GThread system no longer supports errorcheck mutexes.");
 }