[LTP] clock_settime: use POSIX runtime detection for CLOCK_MONOTONIC

[Petr Vorel]

> > New Open POSIX testsuite headers should carry a "Created by: name <email>"
> > line and a brief description of the file's purpose in the copyright block,
> > consistent with the rest of the tree (e.g. helpers.h). Please add both.


> This is correct, I will fix it before merging it.
> Ping @Petr

Replied previously. BTW Agent removed "v2" from the title [1], therefore I did
not reply here because I did not want my reply to endup in old patch version.

It'd be great if Agent just keep the title to not confuse humans (people usually
trust the title instead of parsing In-Reply-To:).

Kind regards,
Petr

[1] https://lore.kernel.org/ltp/20260428144259.3779-1-linuxtestproject.agent@gmail.com/