Explicitly initialize D-Bus GLib threading

When gconf was switched from orbit to dbus it was no longer thread-safe by
default. This behavior can only get back by explicitly initializing dbus-glib's
threads. This issue affects multi-threaded GConf# users like Banshee leading
them to SEGVs.

For more details see:
https://bugzilla.gnome.org/show_bug.cgi?id=683830