mtime: Use mach_absolute_time on Mac OS X instead of gettimeofday.

mach_absolute_time() is a monotic clock, whereas gettimeofday() is not. For that reason setting your computer time while playing a movie won't halt playback anymore.

Seems to fix the dropped frame issue on Mac OS X.
Signed-off-by: Derk-Jan Hartman <hartman@videolan.org>