futhark: introduce the top-level futhark attribute

The fact that futhark is a Haskell package is an implementation detail. To
install it users should just have to specify `futhark` instead of
`haskellPackages.futhark`.

Additionally futhark is overridden with `haskell.lib.justStaticExecutables` to
reduce closure size.

pkgs/tools/audio/mpdsync/default.nix

@@ -0,0 +1,24 @@
+{ stdenv, python2, fetchFromGitHub }:
+with python2.pkgs;
+stdenv.mkDerivation {
+  name = "mpdsync-2017-06-15";
+
+  src = fetchFromGitHub {
+    owner = "alphapapa";
+    repo = "mpdsync";
+    rev = "da90058f44dd9578cc5f2fb96a1fb2b26da40d07";
+    sha256 = "1mfg3ipqj5dvyyqbgp6ia6sc1ja5gmm2c9mfrwx0jw2dl182if6q";
+  };
+
+  pythonPath = [ mpd2 ];
+
+  nativeBuildInputs = [
+    wrapPython
+  ];
+
+  dontBuild = true;
+
+  installPhase = "install -D mpdsync.py $out/bin/mpdsync";
+  postFixup = "wrapPythonPrograms";
+
+}