[tz] Getting current tzdb version in use

[Paul Eggert]

On 07/19/2018 08:39 AM, Howard Hinnant wrote:
> If every version == “unknown”, this strategy will spectacularly fail, and subsequently the std::lib vendor may as well.

Presumably get_tzdb().version should have some way of failing if there 
is no version info, and it should fail if tzdata.zi says '# version 
unknown'. Although this wouldn't solve the other version problems we've 
seen on this thread, it should solve this one.