warn when two input produce same output (and overwrite)

[ygrek]

See ocaml/oasis#114

[gasche]

Thanks. I think that an ocamlbuild-side solution would be interesting, because ocamlbuild has a better view of intermediate targets than oasis can have (without reverse-engineering the build process). A warning to start with is appropriate, because I would not be surprised if racy file production actually happened in some externally-valid ocamlbuild runs today that people would not like to see breaking. (Of course it's better if rules are rewritten to not be racy, and we could consider making this a failure in the future.)

I think that another failure mode that is silent today (while it looks pretty bad) is for a rule with multiple static outputs to actually not produce one of the target. It may or may not make sense to check for this error in the same style as we check for overridden files.