Changeset b279013 for configure.ac

Start of GMainContext code

Add GSources to feed owl_timer and owl_io_dispatch events into the main
loop. Also add a hack so pre-select actions run at all. Glib's main loop
has a hard priority system, so it prefers that prepare/check hooks
return if the event would actually do anything. We probably want to
replace every pre-select action with a dedicated GSource (or maybe a
bunch of g_idle_add calls).

Signals are also racy right now; glib uses select/poll instead of
pselect/ppoll, so you can't actually listen for signals on the same
thread. (In fact, the single-threaded version of g_child_watch_add
doesn't actually work.) That's okay as what we were doing doesn't work
when you add a second thread and thread safety is sort of the point of
this work.

The AIM hack need also be restored.