Inconsistent behavior between custom and simple build types

(Imported from [Trac #716](http://hackage.haskell.org/trac/hackage/ticket/716), reported by @j3h on 2010-07-23)

If a Custom build-type is used, Setup.hs may be built with a different version of Cabal than the one that cabal-install was built with. This can cause hard-to-find (seemingly random) build failures.

Perhaps cabal-install should warn when it runs Setup.hs with a different version of Cabal than the one that it was built with. Alternatively, it could fail if the version of Cabal that cabal-install was built with is not available when compiling the Setup script.