[tz] Add option to act like global-tz

I wangled a bit of free time and wrote the attached proposed patch, which adds an option that causes 'make install' to generate time zone data (TZif files) that are byte-for-byte identical to what Stephen Colebourne's global-tz currently generates. This is done via a few lines of awk script. The idea is to give an option to distros that prefer a separate Zone per ISO 3166 country, just as we already have build-time options for rearguard/vanguard form, TZif truncation, leap seconds, etc. I haven't installed this in the development database. Comments welcome as usual.