The experiment to add `-no_weak_imports` unconditionally has been a bit
of a failure. It's broken more than it's fixed and I don't think we can
have this as a default for Xcode 8.
Add `ENV.no_weak_imports` to be used by formulae authors and make
`ENV.no_weak_imports a no-op (for now).
We may reconsider this behaviour in future.