Cabal fix.

Setup.hs

@@ -1,2 +1,16 @@
 import Distribution.Simple
-main = defaultMainWithHooks defaultUserHooks
+import Distribution.Setup   ( ConfigFlags (..) )
+import System.Directory     ( findExecutable )
+
+main = defaultMainWithHooks (defaultUserHooks { postConf = defaultPostConf })
+    where defaultPostConf args flags lbi
+              = do args' <- fmap (args++) (configToArgs flags)
+                   (postConf defaultUserHooks) args' flags lbi
+
+configToArgs :: ConfigFlags -> IO [String]
+configToArgs (ConfigFlags { configHcPath = Just hcPath })
+    = do exec <- findExecutable hcPath
+         case exec of
+           Just realPath -> return ["--with-ghc="++realPath]
+           Nothing -> return ["--with-ghc="++hcPath]
+configToArgs _ = return []