Download deps automatically if missing when compiling

When running `make` dependencies are not automatically downloaded.

Usually this is not a problem since we can run `make deps compile`.

But if you use https://github.com/extend/erlang.mk you can not do this. Erlang.mk will automagically call `make` and that's it (ref : https://github.com/extend/erlang.mk/blob/master/erlang.mk#L184)

This pull request solves this problem.