mktime.c fixes (part 1 of 6): assume freestanding C89 or better

[Roland McGrath]

> 2003-12-30  Paul Eggert  <eggert@twinsun.com>
> 
> 	* time/mktime.c: Assume freestanding C89 or better.
[...]

I put this one in, it has no potential for controversy.


Thanks,
Roland