bug#27810: macOS runtime feature detection

[Richard Stallman]

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  > >   > We should be able to build a single binary that can detect new OS
  > >   > features at runtime rather than depend on compile‐time macros.
  > > 
  > > One reason to avoid doing it that way
  > > is for the sake of cross-compilation.

  > I’m afraid I don’t understand why it’s a problem for cross
  > compilation.

Autoconf tests that test the compilation environment
will automatically do the right thing in cross-compilation.
That's why we try to implement ALL tests that way.

  > At the moment we don’t even provide any way of building
  > Emacs on one version of macOS that is guaranteed to work on another
  > version of macOS, never mind cross‐compiling the NS port from another
  > platform.

Ideally cross-compiling should work from any platform to any platform
if you have installed suitable cross-compilation tools.  If this
does not work, in principle that is a bug in Emacs.  I am not saying
it is an extremely important bug to fix, but we should not introduce
more of them.

-- 
Dr Richard Stallman
President, Free Software Foundation (gnu.org, fsf.org)
Internet Hall-of-Famer (internethalloffame.org)
Skype: No way! See stallman.org/skype.html.