GNU/Hurd uses BSD-based interfaces, but does not (and won't) provide <sys/sysctl.h>
Doesn't __APPLE__ imply __MACH__ anyway outside of targets that are way out of scope of libc++? Since IIRC libc++ only runs on Darwin and only in usermode, which has the __APPLE__ predefined macro even for stuff like PureDarwin. XNU itself lacks a real libc++, and so do other non-usermode things (providing either simple stubs or very minimal more suited implementations).
Doesn't APPLE imply MACH anyway outside of targets that are way out of scope of libc++?
Well, possibly, it is just not directly related to my patch :)
(also it makes sense to keep it at least as documenting that it's the mach-powered apple OS that provides sys/param.h + sys/sysctl.h).