Add a more sound CLOCK_MONOTONIC check.

* configure.ac now attempts to compile a program using clock_gettime, and if
  successful, enables NN_HAVE_CLOCK_MONOTONIC. It also determines of -lrt should
  be used.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Mark Ellzey added to AUHORS file

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>