Fixed optional dependencies.

I misunderstood the formula syntax.

    depends_on 'libxml2' => [:optional, 'with-xml2']

will build the libxml2 dependency with the 'with-xml2' flag. What I
wanted was to rename the 'with-libxml2' flag on the emacs-mac formula
itself to 'with-xml2' (because emacs itself is using 'with-xml2').

Fixed in this commit (also for the 'with-dbus' flag).

Sorry for the inconvenience.