Fudge to compile libsgp4 DateTime on Mac OS X
Updated libsgp4 uses `clock_gettime` in the new DateTime class - but it's only available in linux. So, this commit adds a preprocessor conditional to use `gettimeofday` instead on OS X (`__MACH__` kernel). gettimeofday's microsecond results are multiplied by 1000 to approximate clock_gettime's nanosecond precision.