[tz] Re: Windows compile failure for iana time zones

Further testing showed that recent patch's use of compound literals also runs afoul of GCC in unusual configurations, and it appears that C23 requires that it use 'constexpr' sometimes but not others. So let's drop that and go to the simpler approach that NetBSD uses. This should also work even with no-longer-supported Microsoft compilers. Proposed further patch installed and attached. I hope this is the last thing we need to change before the 2025a release.