Pushed as 2e951707f505db2d6e3a94a842b4319aa9b07e39, thank you!

I've modified the definition to use the dune-build-system, moved cppo
to native-inputs and modified the names of the inputs in that way:

("cppo" ,ocaml-cppo) -> ("ocaml-cppo" ,ocaml-cppo)

to correspond better to the way we usually declare dependencies. I know
the ocaml.scm file doesn't always respect this convention, but please
try to stick to it anyway :)