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

[Paul Eggert]

On 2022-11-20 12:47, Russ Allbery wrote:
> I only mention it because I was curious whether the underscores were best
> practice.

I put the underscores in due to long habit with public include files. 
But they're not needed in private.h, so I installed the attached.

> (I've seen these C23 attributes before, but I don't yet
> understand how they work.)

These standardize some of the attributes that have long been supported 
by GCC, but using a nicer syntax because the C standardizers weren't 
constrained to use a syntax that can be #defined to empty.
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