Merge: macOS: update realtime lib for Sierra

The latest version of macOS, Sierra, defines the functions `clock_gettime` and `clock_getres` which were previously missing. This PR adds support for Sierra while preserving compatibility with El Capitan using a simple heuristic to either use the system provided functions or define them locally.

Reported-by: Marie-Pier Lessard @mplessard

Pull-Request: #2358
Reviewed-by: Jean-Philippe Caissy <jpcaissy@piji.ca>