When clean fails, "rm -rf _build"

[gildor478]

This feature request has been migrated from artifact #780 on forge.ocamlcore.org. It was assigned to user102.

user102 posted on 2010-10-22 09:35:50:

We should remove the _build directory if something goes wrong. E.g. the _build/myocamlbuild doesn't match the arch (i386 <> amd64).