Elm: fix build by ignoring the build instructions provided by upstream

According to <https://github.com/elm-lang/Elm/issues/384>, Elm's release
archive comes with a Setup.hs that cannot compile an Elm release. Duh!

Replacing the custom Setup.hs file with a dummy version fixes this
issue.

Close <https://github.com/NixOS/nixpkgs/issues/2089>.

pkgs/development/compilers/elm/elm.nix

@@ -23,6 +23,10 @@ cabal.mkDerivation (self: {
     transformers unionFind unorderedContainers
   ];
   doCheck = false;
+  preConfigure = ''
+    rm -f Setup.hs
+    echo -e "import Distribution.Simple\nmain=defaultMain\n" > Setup.hs
+  '';
   meta = {
     homepage = "http://elm-lang.org";
     description = "The Elm language module";