Restrict overmatching MACH ifdef to only trigger on OSX and Mach

[Zopolis4]

Hurd uses a different version of Mach that does not have mach/clock.h. https://www.gnu.org/software/hurd/hurd/porting/guidelines.html

#ifdef __MACH__
Some applications put Apple Darwin-specific code inside #ifdef __MACH__ guards. Such guard is clearly not enough, since not only Apple uses Mach as a kernel. This should be replaced by #if defined(__MACH__) && defined(__APPLE__)