* modules/largefile-required, modules/year2038-required: New modules.
* MODULES.html.sh, doc/largefile.texi, doc/posix-headers/time.texi:
* doc/year2038.texi: Document this.
* m4/largefile.m4: Sync from Autoconf master.
Conditionalize the workaround on AC_SYS_LARGEFILE_REQUIRED rather
than on AC_SYS_YEAR2038 so that we replace older but still
unreleased Autoconf.
(AC_SYS_LARGEFILE_REQUIRED, AC_SYS_YEAR2038_REQUIRED): New macros.