Merge: Fix lib/realtime on osx

A lot of googling to solve two related issues on Mac OS X

* `clock_gettime` and cie is not provided, and one should use the more complex clock_get_time (and cie)
* the POSIX `-lrt` is refused by the linker

close #868

Pull-Request: #1586
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>