Port autogen.sh hook creation to Apple Git

Port autogen.sh hook creation to Apple Git

* autogen.sh: Do not assume that the hook samples exist.
This ports to git version 1.9.3 (Apple Git-50).
For some reason Apple decided to populate the git hooks directory
in a different way from the way that standard Git does it.
The downside is that patch applications won't be checked on Apple,
but that's better than autogen.sh failing.
Problem reported by Sam Steingold in:
http://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00898.html