mktime: do not assume 'long' is wide enough

mktime: do not assume 'long' is wide enough

* time/mktime.c (verify): Move decl up.
(long_int): New type.
(leapyear, ydhms_diff, guess_time_tm, __mktime_internal): Use it,
to remove assumption in the code that 'long' is wide enough to
store year values.  This assumption is not true on x32 and on
some non-glibc platforms.