MPR#6099: Add ocamlbuild tags mirroring command line options

[damiendoligez]

PR transferred from https://caml.inria.fr/mantis/view.php?id=6099 [original reporter: @danmey]

We should allow tags for ocamlbuild:

  pflag ["ocamlbuild"; "library"] "ocamlopt"
    (fun param -> S [A "-ocamlopt"; A param]);

and

<lib/*>: ocamlopt(arm-linux-gnueabi-ocamlopt)

[whitequark]

Triage: same issues as described in https://github.com/ocaml/ocamlbuild/issues/185#issuecomment-283486418.

[gasche]

I think that we should be able to separate command-line options that have a global semantics on the ocamlbuild invocation, and cannot easily be turned into flags, and those that can. In this example, we could give a local semantics to the ocamlc option if we wanted to (and it would be natural to cascade, a tag setting would override an invocation-global setting), but it seems tricky to implement. I think that a tag for lflag, etc. would be more robust and easier to implement (but then it is in competition with command-line-option-specific tags).

[whitequark]

I agree wrt lflag and friends.