make ppx_tools an optional dep in OPAM

[lefessan]

I recently tried to install lwt with 4.04.0+trunk, and it failed because ppx_tools is not yet compatible with that version. However, setting ppx_tools as an optional dep (because I use a copy of the opam repository that I could easily modify...), I could finally install it.

My suggestion:

• set ppx_tools as an optional dependency of lwt in opam
• create a package lwt+ppx, with no sources, but that depends on both lwt and ppx_tools. Packages that depend on the ppx syntax of lwt would then use a dependency on that new package instead of the one on lwt.

[aantron]

Not a bad suggestion, but following it exactly will have to wait, as it will be a breaking change for some users. See also #41.

[Drup]

I don't think that's a good idea, users expect to have the ppx on the relevant version of ocaml+lwt.

@lefessan The right solution for you is to make base-no-ppx available on 4.04, via a pin. It's a simple one liner, and it doesn't bother non-bleeding edge users. See ocaml/opam-repository#6373

[aantron]

@Drup's suggestion is the right way.

I'm not sure how to pin base-no-ppx with a one-liner, but it can be done like this:

mkdir base-no-ppx
echo 'opam-version: "1.2"' > base-no-ppx/opam
opam pin add -y base-no-ppx base-no-ppx

Remove the OCaml version constraint when the editor comes up, and base-no-ppx should install.