Add support for cabal custom-setup

[iliastsi]

Hi,

While trying to package the latest version of semigroupoids (v5.2.1) for Debian, I noticed that the tests failed because of a missing build-dependency on cabal-doctest. The above package has a setup-depends dependency on cabal-doctest but cabal-debian does not translate that into a Debug build dependency. Would it be possible for cabal-debian to support cabal's custom-setup?

Thanks!