GLib 2.32 has been released in March 2012, so we can now require it a
build and run time. This allows us to remove GLib 2.31 conditionals,
mark the Thread.Init as obsolete, and remove all references to
libgthread.
Please note that the API exposed in glib and gio is still from 2.28, but
we will have some API additions in the future. First example is the next
commit, which brings GBytes, an addition in GLib 2.32.
