Test failures with OCaml 4.08

Forwarded from https://bugs.debian.org/939386 by @glondu

Your package diffoscope FTBFS with OCaml 4.08.0.

The failure is due to a test that inspects a .cmi file that is shipped by the source package, using the "ocamlobjinfo" command. This cannot work.

Indeed there is no compatibility guarantee on .cmi (or any other type of object files) between different versions of OCaml. And this incompatibility is what hurts here: the .cmi shipped in the source package was made with OCaml < 4.08.0. The difference in version can be seen with the "file" command.

I have two suggestions:

Handle gracefully the case where ocamlobjinfo rejects an object file because it was not compiled with the same version of OCaml.
Instead of shipping a .cmi file in the source package, generate one at build time.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information