Disable production of shared libraries (.cmxs, .so)

This is needed primarily for MirageOS, but may also be useful for others. We would like a way to disable production of all shared libraries (.cmxs for OCaml, .so for C code) on a per-project (or indeed, for Mirage on a "entire universe") basis.

The only workaround which currently works without requiring any hacks or foreign library changes is to create a custom compiler variant where that compiler is configured with --disable-shared, which propagates down to dune nicely.

That doesn't seem like a hack. It seems like there should be a way to set this option for any context from inside dune.

How about we make it possible to override the -config options for any context from inside dune? This could be done in the workspace file:

(context
 (default
  (config
   (supports_shared _libraries false))))

Originally posted by @rgrinberg in https://github.com/ocaml/dune/issues/3718#issuecomment-712428099