fix(gemma): Almost fix Gemma build by porting an old Elm

This pulls in an old version of Elm from NixOS 17.09 which can still
build the Elm code in Gemma.

However, the Common Lisp build is now broken in some other way.

default.nix

@@ -12,7 +12,7 @@ let
     tazjin = {
       blog = import ./services/tazblog { inherit pkgs; };
       blog_cli = pkgs.callPackage ./tools/blog_cli {};
-      gemma = import ./services/gemma { inherit pkgs; };
+      gemma = pkgs.callPackage ./services/gemma {};
     };
 
     # Third-party projects (either vendored or modified from nixpkgs) go here:
@@ -25,6 +25,15 @@ let
       rev = "a7076c0f83e5c06ea9067b71171859fa2ba8afd9";
       sha256 = "1p4n2ja4ciw3qfskn65ggpy37mvgf2sslxqmqn8s8jjarnqcyfny";
     }) { pkgs = super; };
+
+    # Gemma needs an older version of Elm to be built. Updating it to
+    # the newer version is a lot of effort.
+    elmPackages = (import (super.fetchFromGitHub {
+      owner = "NixOS";
+      repo = "nixpkgs";
+      rev = "14f9ee66e63077539252f8b4550049381a082518";
+      sha256 = "1wn7nmb1cqfk2j91l3rwc6yhimfkzxprb8wknw5wi57yhq9m6lv1";
+    }) {}).elmPackages;
   };
 
   # The pinned commit here is identical to the public nixery.dev

services/gemma/default.nix

@@ -1,15 +1,9 @@
-with import <nixpkgs> {};
+{ stdenv, sbcl, lispPackages, elmPackages, pkgconfig }:
 
 stdenv.mkDerivation rec {
   name = "gemma";
-
   src = ./.;
 
-  # For the time being, Gemma can not be built because it requires an
-  # older version of Elm than what is available and Elm upgrade paths
-  # are painful.
-  broken = true;
-
   buildInputs = with lispPackages; [
     sbcl
     quicklisp
@@ -50,5 +44,9 @@ stdenv.mkDerivation rec {
     description = "Tool for tracking recurring tasks";
     homepage    = "https://github.com/tazjin/gemma";
     license     = licenses.gpl3;
+
+    # For the time being, Gemma can not be built because the Lisp build process
+    # broke in some way I haven't had time to debug.
+    broken = true;
   };
 }