Use import list to hide new System.Exit.die

Starting with GHC 7.10 (and soon GHC HEAD), System.Exit exports the new `die`
which would clash with Cabal's own `die`. See also

  https://ghc.haskell.org/trac/ghc/ticket/9016

For more details.

Cabal/Distribution/Simple.hs

@@ -134,7 +134,7 @@ import Distribution.Text
 import System.Environment(getArgs, getProgName)
 import System.Directory(removeFile, doesFileExist,
                         doesDirectoryExist, removeDirectoryRecursive)
-import System.Exit
+import System.Exit       (exitWith,ExitCode(..))
 import System.IO.Error   (isDoesNotExistError)
 import Control.Exception (throwIO)
 import Distribution.Compat.Environment (getEnvironment)