configure.ac: Bump Gtk+ requirement to 2.12.0 for gtk_range_set_fill_level (NB: this only affects compilation of some...

Original commit message from CVS:
* configure.ac:
Bump Gtk+ requirement to 2.12.0 for gtk_range_set_fill_level
(NB: this only affects compilation of some of the examples).
Remove some configure.ac cruft that's not needed any longer.