haskell-HTTP: don't process these sources with Haddock from GHC 6.10.x
haskell-HTTP: require a recent Cabal for building this package
haskell-zlib: require a recent Cabal for building this package
haskell-jailbreak-cabal: fix build with GHC 6.x
cabal-install: fix builds with GHC 6.x
haskell-packages.nix: cosmetic