regex: fix support for overriding "gnulib" Problem reported by Pádraig Brady in: https://lists.gnu.org/r/bug-gnulib/2025-09/msg00104.html * lib/regex_internal.h [!_LIBC]: Simplify by including gettext.h rather than doing things by hand and mishandling GNULIB_TEXT_DOMAIN. (gettext_noop) [!_LIBC]: Remove, since gettext.h does that now. * modules/regex (Depends-on): Add gettext-h.