largefile: port to C++

largefile: port to C++

This patch is mostly taken from Autoconf master.
* m4/largefile.m4 (AC_SYS_YEAR2038_RECOMMENDED):
Undefine if unpatched Autoconf 2.72 or earlier, so that
later code will redefine it.
The remaining part of this patch is from Autoconf master.
(_AC_SYS_YEAR2038_PROBE, _AC_SYS_LARGEFILE_PROBE):
Put "$CCFLAGS" in diagnostics, not "$CC".
(_AC_SYS_LARGEFILE_OPTIONS): Omit -n32.
(AC_SYS_LARGEFILE_PROBE): Fiddle with CPPFLAGS, not CC.
Do not worry about -n32.