visitors depends on cppo_ocamlbuild

[gasche]

This missing dependency was not noticed before as it was a transitive
dependency through ppx_deriving. Now ppx_deriving doesn't depend on it
anymore, so builds of visitors in an empty environment (typically the
CI) fail:

https://ci.ocamllabs.io/log/saved/docker-run-3fd09aeb82dc8ddea48a0bec2d629251/e25f76f9a648739a0a93bcc6d52c5fb52cf4cae4

[camelus]

🌤️ opam-lint warnings 93da415

• visitors.20170828 has some warnings:

  • warning 47: Synopsis should start with a capital and not end with a dot

• visitors.20171124 has some warnings:

  • warning 47: Synopsis should start with a capital and not end with a dot

• visitors.20180306 has some warnings:

  • warning 47: Synopsis should start with a capital and not end with a dot

• visitors.20180513 has some warnings:

  • warning 47: Synopsis should start with a capital and not end with a dot

• These packages passed lint tests: visitors.20170404, visitors.20170420, visitors.20170725

---

☀️ Installability check (11112 → 11112)

[gasche]

There is a build failure in clangml.4.0.0beta1 (cc @thierry-martinez), which looks related to visitors but unrelated to the present change.

+ /home/opam/.opam/4.07/bin/dune "build" "-p" "clangml" "-j" "71" (CWD=/home/opam/.opam/4.07/.opam-switch/build/clangml.4.0.0beta1)
-          ppx clangml/clang__ast.pp.ml (exit 1)
- (cd _build/default && .ppx/4fc829e6430080dcaa34c1d4da51aedf/ppx.exe --cookie 'library-name="clangml"' -o clangml/clang__ast.pp.ml --impl clangml/clang__ast.ml --dump-ast)
- File "clangml/clang__ast.ml", line 2170, characters 6-14:
- Error: Cannot locate deriver visitors

Otherwise everything is fine. Merging.