[tz] [PROPOSED] Use C23 [[maybe_unused]] if available

[Russ Allbery]

There's a minor inconsistency here with the Subject header, which says
[[maybe_unused]], and the patch, which says [[__maybe_unused__]].

I only mention it because I was curious whether the underscores were best
practice.  (I've seen these C23 attributes before, but I don't yet
understand how they work.)

Google results seem to mostly not use the underscores.

-- 
Russ Allbery (eagle at eyrie.org)             <https://www.eyrie.org/~eagle/>