Use gettimeofday() on Mac OS X.

Mac OS X lacks CLOCK_MONOTONIC as well as clock_gettime and
pthread_condattr_setclock.

There is some concern that this might not be safe on other
platforms that also happen to lack CLOCK_MONOTONIC or that
there might be better alternatives, so we only do this on
Mac OS X for now.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Use CLOCK_REALTIME when CLOCK_MONOTONIC is not available

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