[tz] problems with unsigned 32-bit time_t coming up

I vaguely recalled someone reporting trouble with the tz database on NetBSD or some other BSD in the past few months. The gist of it was that this platform has 32-bit 'long' and 64-bit 'time_t', and the tz code didn't play nicely on such a system. To try to catch this sort of thing I'd like to add support for trying different time_t types, even if they disagree with the system's time_t. I'll send a couple of proposed patches to the list to do that. I'll then follow up with a report of a bug in the reference implementation that is discovered by the patches, when configured to run with an unsigned 32-bit time_t. This isn't the NetBSD problem, but hey! at least there's an easy-to-run test case that illustrates the bug.