bug#30364: 26.0.91; thread crash on macos

[Aaron Jensen]

On Wed, Feb 28, 2018 at 9:38 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> But that is a macOS only problem, because the default stack size is
> ridiculously small.  The fix that got installed, OTOH, is much more
> general.
>
> If you want to make a macOS-only change that enlarges its stack we
> give to threads, I can live with that on emacs-26, provided that the
> fix only affects threads other than the main one.

So wrap the new code in an #if DARWIN_OS?