Relax g_thread_init() requirements

We now allow g_thread_init(NULL) to be called after other glib calls (with
some minor limitations). This is mainly a documentation change as this
really was already possible.

We also allow g_thread_init() to be called multiple times. Only the
first call actually initializes the threading system, further calls
are ignored (but print a warning if the argument is not NULL).

https://bugzilla.gnome.org/show_bug.cgi?id=606775

gthread/gthread-impl.c

@@ -294,7 +294,12 @@ g_thread_init (GThreadFunctions* init)
   gboolean supported;
 
   if (thread_system_already_initialized)
-    g_error ("GThread system may only be initialized once.");
+    {
+      if (init != NULL)
+	g_warning ("GThread system already initialized, ignoring custom thread implementation.");
+
+      return;
+    }
 
   thread_system_already_initialized = TRUE;