Set MacOS platform version in one place

[chalcolith]

This PR specifies the MacOS platform version (currently "13.0.0") in only one place, and uses CMake variables and C/C++ defines to set it appropriately.

Fixes #4469

[SeanTAllen]

why do we need STR2 instead of only STR(X) #x?

[jemc]

Looks good, other than sharing Sean's question about the macro

[chalcolith]

That's how the C preprocessor works: https://gcc.gnu.org/onlinedocs/gcc-3.4.3/cpp/Stringification.html

"If you want to stringify the result of expansion of a macro argument, you have to use two levels of macros.

     #define xstr(s) str(s)
     #define str(s) #s
     #define foo 4
     str (foo)
          ==> "foo"
     xstr (foo)
          ==> xstr (4)
          ==> str (4)
          ==> "4"

s is stringified when it is used in str, so it is not macro-expanded first. But s is an ordinary argument to xstr, so it is completely macro-expanded before xstr itself is expanded (see Argument Prescan). Therefore, by the time str gets to its argument, it has already been macro-expanded."