Update emacs-mac.rb

I noticed installing did not build with libraries installed via Homebrew (i.e. I wondered why it wasn't build with libxml2 although I had installed it with brew). Adding `pkg-config` as a build dependency fixed that.

Also marked `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.