[tz] make rearguard_tarballs fails on macOS

[Paul Eggert]

On 10/16/20 9:09 PM, Deborah Goldsmith wrote:
> According to Wikipedia Darwin is not POSIX-compatible but is compatible with the Single UNIX Specification, version 3.

The Single UNIX Specification version 3 is the same as POSIX-2001, so I guess 
2001 is as far as Apple has wanted to go, at least so far.

Wikipedia says AIX and HP-UX also don't go past POSIX-2001, so today's fix 
should be helpful on those old OSes too.