[tz] Getting current tzdb version in use

[Paul Eggert]

On 07/19/2018 08:50 AM, lester at lsces.co.uk wrote:
> The bottom line is that currently we have no way of knowing just what data is 
> being supplied. Even if the data matches the raw tz version, it may or may not 
> include the pre-1970 <tel:1970> data archived to backzone.

Yes, and this is partly why I have resisted putting version information into the 
installed files. Version info is typically not available unless you've merely 
cloned the development Git repository and have made no change to the data or to 
the 'make' parameters that select or alter the data, a situation that is 
reasonably rare in practice. It would take quite a bit of effort to get reliable 
version information in real installations, and nobody knows exactly what would 
be entailed in that effort. Since many people have requested version info I had 
compromised by putting a version string into tzdata.zi as a sort of experiment. 
The experiment has clearly failed so far.

That being said, we can improve things a bit by putting into tzdata.zi 
information about whether the file 'backward' was used, as well as whether the 
(dicey) file 'backzone' and the (obsolete) file 'pacificnew' were used. We can 
also record whether the rearguard (or vanguard) data format was selected. 
Proposed patch attached.

Harold, if you're changing your C++ library to look for "unknown", this patch 
might append some stuff to that "unknown" string.

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