CPUTimer: switch to clock_gettime

  - switch from gettimeofday/getrusage to clock_gettime,
    with the same speed but better accuracy (at least on x86_64 Linux)
  - switch from clock_gettime(CLOCK_REALTIME) to clock_gettime(CLOCK_MONOTONIC)
    with a very similar performance, but conceptually more sound
Merge pull request #4667 from fwyzard/CPUTimer_use_clock_gettime

CPUTimer: switch to clock_gettime