* zdump.c: Tune performance of -c with large numbers on 64-bit hosts.

On my platform (Ubuntu 13.04 x86-64, Xeon E3-1225 V2), this sped up
'zdump -V -c 2147483647,2147483648 America/Los_Angeles'
from 13.880 to 0.000 seconds user+system time.
(INTMAX_MAX): Define if the system doesn't.
(SECSPER400YEARS): New macro.
(SECSPER400YEARS_FITS): New constant.
(yeartot): Speed up by using division instead of
repeated subtraction.