nl_langinfo: Work around nl_langinfo multithread-safety bug on macOS 26. * m4/nl_langinfo.m4 (gl_FUNC_NL_LANGINFO): Set NL_LANGINFO_MTSAFE to 0 on macOS. * lib/nl_langinfo.c (ITEMS): Define appropriately on macOS. * doc/posix-functions/nl_langinfo.texi: Document the macOS bug.