Update from the latest y2038.

Update from the latest y2038.

Include the ability to use the system functions.  This speeds up localtime()
several times when the time is < 32 bits.

Don't bother using the system gmtime() as it will probably just have bugs.
We need to wait for config to give us LOCALTIME_MIN/MAX but I think assuming
time_t is at least 32 bits is good enough for now.

This restores the full performance of localtime() for < 32 bit numbers
and gmtime() is now only 10% slower.