Extend ghcHEAD derivation rather than make a new derivation (#486)
Extend ghcHEAD derivation rather than make a new derivation (#486)

This allows you to use configurePhase/buildPhase as usual and
set the right configureFlags rather than end up with a broken
executable.
sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,snowleopard/shaking-up-ghc,sdiehl/ghc,bgamari/shaking-up-ghc,snowleopard/hadrian,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc