feature(melange): allow to set compiler flags just for melange or for ocaml

[jchavarri]

Currently, flags allow to set both OCaml and Melange flags. But if one wants to set flags for either Melange or OCaml libraries in a shared code setup, it's not possible:

• For OCaml only flags, one can set ocamlc_flags combined with ocamlopt_flags
• For Melange only flags, it is not possible

It would be nice to have a way to define flags conditionally, e.g. (flags (if (<> %{modes} melange) ...)). Or alternatively, leave flags only for OCaml compilation, and add a new field melange_flags.

[jchavarri]

Actually, the description above is not accurate.

For OCaml only flags, one can set ocamlc_flags combined with ocamlopt_flags

At the moment, Melange inherits the flags set for byte compilation (see related PR #6548). So there is no way to set flags for byte compilation without setting them for Melange as well.

I think it could be useful in the future to have this kind of ability. A way to fix this could be:

• Add new flag melc_flags that would only apply to Melange
• Stop inheriting flags from ocamlc_flags in Melange compilation
• Leave flags behavior as it currently is: it would affect all melange, byte and native compilation

This should be much easier to implement than the conditionally enabled flags mentioned earlier.