Enable threads in g_type_init()

This means threads will be supported for all gobject libraries/applications
and initialized early enough to not cause any problems.

This solves the problem of libraries needing threadsafety. Previosly
they just called g_threads_init() anyway, which often works but sometimes
breaks in unexpected ways.

See this thread for more details:
http://mail.gnome.org/archives/gtk-devel-list/2009-November/msg00208.html

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

docs/reference/glib/tmpl/threads.sgml

@@ -48,6 +48,14 @@ However, many libraries above glib does not support late initialization of
 threads, so doing this should be avoided if possible.
 </para>
 
+<para>
+Please note that since version 2.24 the GObject initialization
+function g_type_init() initializes threads (with a %NULL argument), so
+most applications, including those using Gtk+ will run with threads
+enabled. If you want a special thread implementation, make sure you
+call g_thread_init() before g_type_init() is called.
+</para>
+
 <para>
 After calling g_thread_init(), GLib is completely
 thread safe (all global data is automatically locked), but individual