emacs: enable vfork on darwin

Issue described here:
https://lists.gnu.org/archive/html/bug-gnu-emacs/2017-04/msg00201.html

In particular fixes lagging magit, as described here:
https://magit.vc/manual/magit/MacOS-Performance.html

The .patch file is taken from the reference there.

The fix is in Emacs master, so this patch should be removed when switching to
Emacs 26.1.