bug#27810: macOS runtime feature detection

[Eli Zaretskii]

> Date: Sun, 30 Jul 2017 13:12:59 +0100
> From: Alan Third <alan@idiocy.org>
> Cc: 27810@debbugs.gnu.org, Eli Zaretskii <eliz@gnu.org>
> 
> Should I stop working on this? I honestly don’t believe it will have
> any effect on cross compilation, and the default is to not include the
> feature detection code unless you ask for it explicitly.

As long as cross-compilation is not affected, there's no reasons for
you to stop working on this.  So please go ahead, just make sure
version-dependent stuff is only used when necessary, i.e. when there
are no reasonable fallbacks that are back-compatible.

Thanks.