threadlib: Omit unsupported configure option. Reported by Michael Osipov <michael.osipov@innomotics.com> in <https://lists.gnu.org/archive/html/bug-gettext/2025-09/msg00009.html>. * m4/threadlib.m4 (gl_THREADLIB_EARLY_BODY): If libtool is in use, don't display the --disable-threads option.