[LTP] clock_settime: use POSIX runtime detection for CLOCK_MONOTONIC

[Andrea Cervesato]

> 
> 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

--
Andrea Cervesato
SUSE QE Automation Engineer Linux
andrea.cervesato@suse.com