[tz] Is $TZ valid?

[Paul Gilmartin via tz]

Simple task.  Verify that a user-supplied TZ value is valid.

So,I imagined, tzset() || perror();

Alas, tzset neither returns status nor sets  errno.

stat( "/usr/share/zoneinfo/$TZ" )?  But:
o I'd like to support systems not supporting tzdata.
  (I know one important one.)
o I'd like to support/verify POSIX format
  e.g.: "MST7MDT,M3.2.0,M11.1.0"

Any ideas?  This must have been done before.

--
Thanks,
gil