Inconsistent behavior between custom and simple build types

[j3h]

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.

[duncan]

I guess we could warn. In general we do need to be able to build using a different Cabal version, even for build-type Simple.

The current behaviour is to use a Cabal lib version that is the same major version as the version that cabal-install was built with, and otherwise to use whatever is the latest available version that meets the version constraints specified in the .cabal file.