configure: Set DATADIRNAME from m4/gettext.m4
gnome_language_has_translations() checks GNOMELOCALEDIR and GNOMELOCALEDIR is configured by DATADIRNAME. If DATADIRNAME is NULL, gnome_get_all_locales() would not return locales.
gnome_language_has_translations() checks GNOMELOCALEDIR and GNOMELOCALEDIR is configured by DATADIRNAME. If DATADIRNAME is NULL, gnome_get_all_locales() would not return locales.