libgettextpo: Make function po_format_list() multithread-safe.
* autogen.sh (GNULIB_MODULES_LIBGETTEXTPO): Add 'once'.
* gettext-tools/libgettextpo/gettext-po.c: Include glthread/once.h.
(all_formats): New variable.
(all_formats_init): New function, extracted from po_format_list.
(all_formats_init_once): New variable.
(po_format_list): Invoke all_formats_init once.
