Improve year2038, largefile option processing

Improve year2038, largefile option processing

* lib/autoconf/specific.m4 (_AC_SYS_YEAR2038_PROBE)
(_AC_SYS_YEAR2038_ENABLE, _AC_SYS_YEAR2038_OPT_IN)
(AC_SYS_YEAR2038, _AC_SYS_LARGEFILE_PROBE)
(_AC_SYS_LARGEFILE_ENABLE): Do not use enable_largefile to record
whether largefile is required, as ‘./configure
--disable-largefile’ sets enable_largefile=no even if largefile is
required and this disables largefile.  Instead, use a separate
shell variable ac_largefile_required and test it as well.
Similarly for enable_year2038.
(_AC_SYS_LARGEFILE_ENABLE): Omit --disable-largefile help string
if year2038 is required, since largefile is a prereq for year2038.