[tz] Getting current tzdb version in use

[Paul Eggert]

Paul G wrote:

> I can tell you a few places I've found need for the version:

Thanks for reporting those war stories; they're helpful, and tend to confirm my 
feeling that the version number can be useful in diagnosing configuration 
mistakes. One comment:

> 3. It can be useful when you have the choice of more than one source for time zone data and want to try and ascertain which one is more recent and use that one.

Unfortunately it's not that simple, because in some cases the sources can 
disagree but neither will be "more recent"; for example, both might be based on 
2018e but with different local patches applied. This is partly why I'm skeptical 
about attempts to use version information for anything important other than 
"report this string to the users, and let them figure it out".

> At this point, of course, I think the battle in the tz project is won - tzdata.zi exists and last I checked `make install` installs it into `/usr/share/zoneinfo`, so now it's time to get system distributors to make sure they include it in their distributions, I guess.

Plus, distributors need to include something better than "# version unknown", 
which is what Red Hat is doing unfortunately. See:

https://bugzilla.redhat.com/show_bug.cgi?id=1604030