Benchmark should use a time source with sub-second resolution

[mejedi]

The benchmark uses time() from time.h to measure elapsed time. It measures time with 1 second resolution. As certain runs complete in 6 seconds or less, the not so fine grained time source introduces significant inaccuracy.

[shadowofneptune]

I agree. I had chosen time() due to its portability, but it's not as good as it could be. Which timekeeping system would you recommend?