emacs: on darwin, do not build Cocoa app (#6882)

Building emacs on darwin throws an error when trying to build an Emacs
app in the nextstep/Emacs.app path of the build tree. For now, disable
building this app.

It's possible to enable building the app also; Homebrew offers options
to this effect, and also adds Mac-specific options for starting the
emacs daemon. However, for the sake of simplicity and getting a
workable up-to-date emacs installation on my machine as quickly as
possible, this commit focuses on a minimal viable modification.