Revise the use of macOS predefined macro

Quoted from "Porting UNIX/Linux Applications to OS X,"[1]
* macro __MACH__ is defined if Mach system calls are supported;
* macro __APPLE__ is defined in any Apple computer.

__MACH__ is not specific to macOS since GNU/Hurd runs on a Mach-based
microkernel (gnumach) [2]. __MACH__ is defined by the compiler,
leading to potential confusions. The solution is just changing the
checked identifier (i.e. __APPLE__), so it is really used only on
macOS.

[1] https://developer.apple.com/library/archive/documentation/Porting/Conceptual/PortingUnix/compiling/compiling.html
[2] https://www.gnu.org/software/hurd/microkernel/mach/gnumach.html

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