mktime: avoid problems on NetBSD 5 / i386

mktime: avoid problems on NetBSD 5 / i386

* lib/mktime.c (long_int): New type.  This works around a problem
on NetBSD 5 / i386, where 'long int' and 'int' are both 32 bits
but time_t is 64 bits, and where I expect the existing code is
wrong in some cases.
(leapyear, ydhms_diff, guess_time_tm, __mktime_internal): Use it.
(ydhms_diff): Bring back the compile-time check for wide-enough
year and yday.