configure.ac: Require GLib 2.12.

configure.ac: Require GLib 2.12.

Original commit message from CVS:
* configure.ac:
Require GLib 2.12.
* gst/glib-compat-private.h:
* gst/gstcaps.c: (gst_caps_new_empty), (_gst_caps_free):
* gst/gstclock.c: (gst_clock_entry_new), (_gst_clock_id_free):
Unconditionally use GSlice for allocation.
* gst/gstpoll.c: (gst_poll_new), (gst_poll_free):
* gst/gstsegment.c: (gst_segment_new), (gst_segment_free):
* gst/gststructure.c: (gst_structure_id_empty_new_with_size),
(gst_structure_free):
Use GSlice for allocation.