clock_gettime is available on Linux

We should be using this function instead, which is potentially more
efficient and doesn't require us calculating 'nanoseconds' from a struct
timeval

Change-Id: Iff89a8dae789575b51b5af7c8037b536e20ebf09
Reviewed-on: http://review.couchbase.org/43536
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>