Emacs HEAD no longer provides autogen/copy_autogen

They feel autotools are now sufficiently wide-spread and up-to-date that they no longer want to support this route. Instead, on Emacs HEAD we now need to run autogen.sh ourselves.

I'm not sure which versions of Xcode provide new-enough autotools, though I could possibly figure that out and add some information to the formula caveats if you'd like.

See also:
* http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=c3b8fa217f0c72c6e1494ccfce54cc1ddda58cc7
* http://lists.gnu.org/archive/html/emacs-devel/2013-10/msg00806.html