Revert "Use upstream gettext instead the glib one"

This causes several problems:
- Compilation in FreeBSD with --enable-gtk-doc broke
- Modules that still use the AM_GLIB_GNU_GETTEXT macro
  doesnt compile anymore because /usr/share/glib-2.0/gettext
  is not filled with the correct files, as this was done in
  the glib custom po/Makefile.in.in

See https://bugzilla.gnome.org/show_bug.cgi?id=622991

This reverts commit e5c75237.