HPCC-11282 OSX build error - sem_timedwait not defined

Go back to old semaphore code on OSX

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5751 from richardkchapman/osx-semaphore

HPCC-11282 OSX build error - sem_timedwait not defined

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>