melange: expose a mechanism to enable conditional compilation

[jchavarri]

Desired Behavior

While virtual libraries are a great mechanism to support conditional compilation, and Melange already supports them, sometimes it is useful to introduce branching at the module level.

For example, cppo allows to check for defined values to conditionally compile some code.

One could generate a cppo rule like:

(rule
 (targets foo.ml)
 (deps
  (:first-dep foo.cppo.ml))
 (action
  (run %{bin:cppo} -D MELANGE %{first-dep} -o %{targets})))

Some other analog rule would be defined for native, but without the -D MELANGE part.

Unfortunately at the moment, it is not possible to express from a dune file which rules are applied only for Melange targets, or only for OCaml targets.

A similar feature was added for foreign stubs in #5649.

cc @davesnx @anmonteiro

[anmonteiro]

I believe this has been superseded by #10222. Please reopen if not.