configure: check for GTK# before GIO#

GTK# macro checks for GLib#. It's better to raise this dependency
before GIO# because it's a bit less confusing (as for v3.0 of the
gtk-sharp bindings, gio-sharp is included in them rather than its
own separate repository as it used to happen for v2.x), as chances
are that by satisfying glib# the user is satisfying all three.