configure: Require GLib >= 2.24

All other modules require this already and nobody is testing with
older versions anyway.

configure.ac

@@ -207,7 +207,7 @@ AC_CHECK_TYPE([struct ip_mreqn], [
 dnl *** checks for dependency libraries ***
 
 dnl GLib is required
-AG_GST_GLIB_CHECK([2.20])
+AG_GST_GLIB_CHECK([2.24])
 PKG_CHECK_MODULES(GIO, [ gio-2.0 >= 2.20 ], , AC_MSG_ERROR([gio is required]))
 
 dnl Orc