Use gettimeofday() on Mac OS X.
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 <4dd6061be1198639e8b05ce4fd5ead7a0dcaa0f4@250bpm.com>

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