Add the alpha feature: dyncomp

I don't know if it's the right way to do it. It's also maybe a patch too quickly done but at least you can comment it.

This « feature » is used (at least) in lwt and tyxml 3.0.0 and due to the use of ```Topdirs```, we can't re-setup it with oasis >= 0.4.0 (without modifying by hand the setup.ml).

cc @Drup