Make threads mandatory

Make threads mandatory

G_THREADS_ENABLED still exists, but is always defined. It is still
possible to use libglib without threads, but gobject (and everything
above it) is now guaranteed to be using threads (as, in fact, it was
before, since it was accidentally impossible to compile with
--disable-threads).

https://bugzilla.gnome.org/show_bug.cgi?id=616754