I am not up to speed on the rest of the changes, but the main goal of having those dependencies for tha mail_app.js target was to reduce impacting others if they were doing a `make install-gaia APP=something` development workflow -- we did not want the email build stuff to be run every time for that case, just once and not redo it if nothing changed in the email area.

Perhaps this changeset also fixes it so that the make work for other apps are not run in the APP=something case? If that is true, then that answers the question above, it would not be run, and so we did not need to have those other dependencies?

Although in that case I wonder, if people are doing multiple `make install-gaia` calls for some reason, the above meant avoiding work in that case too. I am not sure how common repeated `install-gaia` is without an APP=.