Change to "stack ghci" cabal_macros.h munging

This way, CURRENT_PACKAGE_KEY will be undefined, causing an error,
rather than just being the last set value.

Author: mgsloan

src/Stack/Ghci.hs

@@ -490,7 +490,7 @@ preprocessCabalMacros pkgs out = liftIO $ do
     let fps = nubOrd (concatMap (mapMaybe (bioCabalMacros . snd) . ghciPkgOpts) pkgs)
     files <- mapM (S8.readFile . toFilePath) fps
     if null files then return [] else do
-        S8.writeFile (toFilePath out) $ S8.intercalate "\n#undef CURRENT_PACKAGE_KEY\n" files
+        S8.writeFile (toFilePath out) $ S8.concat $ map (<> "\n#undef CURRENT_PACKAGE_KEY\n") files
         return ["-optP-include", "-optP" <> toFilePath out]
 
 setScriptPerms :: MonadIO m => FilePath -> m ()