cabal ignores the 'cabal-version' field when solving for setup dependencies

[sebright]

This can cause the build to fail when the setup-depends range for Cabal is less restrictive than the cabal-version for a given package. See https://github.com/haskell/cabal/pull/3723#issuecomment-243337073.

The current behavior is technically correct, because it is possible for a setup script to depend on one version of Cabal and implement the command interface from a different version. I think it would be better to prevent build failures in the more common case, though.

One solution is to update the setup-depends constraint on Cabal to reflect cabal-version before dependency solving.