Controlling flags for ocamldep (e.g. for specifying keywords)

[pmundkur]

There will likely be a need to allow specification of flags to ocamldep, depending on the resolution of ocaml/ocaml#13764. Currently, as per the docs, flags can only be specified for ocamlc and ocamlopt.

Alternatively, there could be a special keywords construct that is applied to all tools that use the ocaml lexer/parser version 5.3.0 or later. However, this might collide with keywords being specified in flags.

[maiste]

Thanks for reporting.
Let's see the outcome of the issue on ocaml/ocaml first 👍