configure: earlier check for glib (bgo#686671)

The macros AM_PATH_GLIB_2_0 and AM_GLIB_GNU_GETTEXT come with glib,
so autoconf is not smart enough to figure out the precedence of
an earlier PKG_CHECK_MODULES(GLIB...), so we now use m4_pattern_allow
to white-list them.

Similar fix to the one for bgo#604416:
http://git.gnome.org/browse/banshee/commit/?id=671eb0302fe2c0dfde2674696396d3effea3746e