Cabal stores ghc version specific details in `dist/build/autogen`

[jacobstanley]

We should detect that the ghc version has changed since the last build, and blow this directory away

[jacobstanley]

/cc @tmcgilchrist I think may have been what was happening to you?

mafia clean will fix it, but it would be good to do it automatically

[tmcgilchrist]

As a work around I've hardcoded my shell to use 7.8.4

[tmcgilchrist]

https://github.com/haskell/cabal/issues/5661