Update emacs-mac.rb

Marking `automake` as a build dependency instead of a general dependency.

Added `pkg-config` as a build dependency so an install will locate libraries installed via Homebrew.

Added libxml2, dbus, gnutls, and imagemagick as optional dependencies to make it easier to get them installed.

Signed-off-by: Arne Jørgensen <arne@arnested.dk>
Update emacs-mac.rb

Fixed name of `d-bus` dependency.
Change option for d-bus to --with-dbus.

So it better mimics the options to Emacs itself.
Listed optional dependencies in alphabetic order.
Merge pull request #11 from arnested/pkg-config

Update emacs-mac.rb