[Wait for dune 2.5] Add compatibility with ocaml 4.02.3

[voodoos]

This is needed for use with merlin .

• Tests are disabled if OCaml version is < 4.04.0
• The result package is used to ensure compat with OCaml < 4.03.0.

This necessitate a future version of dune: probably 2.5.

[ghost]

Normally you should be able to merge this yourself (once the new dune is released).