Do not traverse _opam by default

[Leonidas-from-XIV]

Kind-of, sort-of related to #257 I think ocamlbuild should know about local switches in _opam and not traverse these folders. As it is projects using oasis and ocamlbuild can't be built using local switches out of the box because of hygiene errors.

I encountered this issue when trying to build https://github.com/andrenth/ocaml-mariadb and it could most likely be fixed by adding _opam to the default no-traverse paths that OASIS generates but fixing up ocamlbuild to do it would be probably the more universal solution.

[Armael]

It looks like this is already fixed in gasche@7fd0fae , which is included in the last ocamlbuild releases. Are you sure your ocamlbuild is up-to-date?

[Leonidas-from-XIV]

Maybe it wasn't, because I re-created what I've been doing and now it seems to be working just fine. Maybe it picked an older ocamlbuild last time. Thanks for looking into it! I'll reopen if I manage to reproduce it with 0.12.0.