dune-project: require OCaml 4.08 or later
dune-project: require OCaml 4.08 or later

The hacks used to build on OCaml < 4.08 are only applied via makefiles.
Adjust the minimal required version to 4.08.

This change has an effect only if generate_opam_files is set to true.

Signed-off-by: Olaf Hering <ded022db89399a060a39da31922f173c6556c2d9@aepfle.de>

paulp/unison