Allow compiling against "dev" version of Coq

This matches e.g. Iris allowing a "dev" version of std++: You can install a "dev" version to test stuff, but then you are responsible for making sure that these versions actually work together. We rely on that when testing things against Iris master every night, for which purpose we install Iris master as "dev" version.

Cc @jjourdan who, IIRC, argue for removing the dev version some time ago.

If we accept this here, I will send similar patches to (some of?) our other projects.

changed the description

@janno mentioned on Mattermost that there also was a problem with opam updating coq to the dev version on make build-dep. That infrastructure has changed since then, so this problem may no longer be present.

I'm not sure what I said exactly, but here is what I think is important: it should be possible to install the dev version of Iris using a non-dev version of Coq. And, possibly, similarly for a non-dev version of SSR.

it should be possible to install the dev version of Iris using a non-dev version of Coq. And, possibly, similarly for a non-dev version of SSR.

This patch adds more versions of Coq that can be used. So, of course, non-dev versions will keep working.

mentioned in commit 2b35253e

merged

Merging because JH's requirements are satisfied. (RK ack'ed privately.)