configure: Invoke AM_GNU_GETTEXT by hand.

The Gnulib "gettext" module does not invoke AM_GNU_GETTEXT, because
gnulib-tool uses "sed" to filter it out.  So we must invoke it
ourselves.

John Darrington narrowed the problem down to the commit b54a5702b6
"Fix up build following dropping ulc-width-linebreaks module" that
started using the gettext Gnulib module.

Bug #33083.
Reported by Mindaugus.