cabal should lint ~/.cabal/config for compat with selected GHC

With GHC 7.8, we need `shared: True` in the cabal config. However, historically `shared: False` has been explicitly set in people's cabal configs for various reasons.

This is actually a really really recurrent problem as people upgrade to GHC 7.8 and cabal 1.18, and I have to suggest `rm ~/.cabal/config ; cabal update` as my quick fix for folks.

It would be really nice (and best) if on systems that need dynamic too / shared / etc, if cabal install would holler "shared: False is possibly a problem" or the like.

Maybe there should be a "cabal doctor" or "cabal lint", I just want to not have this gotcha be in my support load.