build: don't use AM_GNU_GETTEXT

AM_GNU_GETTEXT can no longer be used with IT_PROG_INTLTOOL.

Same problem/fix as https://bugzilla.gnome.org/show_bug.cgi?id=706828;
see https://bugzilla.gnome.org/show_bug.cgi?id=705365#c7 for more
details.

https://bugzilla.gnome.org/show_bug.cgi?id=709203