emacs-irony: prebuild irony-server executable

This has melpaBuild compile the irony-server executable automatically.
This means each user of the irony executable doesn’t have to wait for
it to compile on each new use. This depends on this PR to work
correctly:

https://github.com/Sarcasm/irony-mode/pull/537

pkgs/top-level/emacs-packages.nix

@@ -288,6 +288,45 @@ let
 
   icicles = callPackage ../applications/editors/emacs-modes/icicles { };
 
+  irony = melpaBuild rec {
+    pname = "irony";
+    ename = "irony";
+    version = "20190516";
+    src = fetchFromGitHub {
+      owner = "Sarcasm";
+      repo = "irony-mode";
+      rev = "c3ae899b61124a747ebafc705086345e460ac08e";
+      sha256 = "06ld83vzyklfmrfi6pp893mvlnhacv9if75c9pbipjvy6nwfb63r";
+    };
+    recipe = fetchurl {
+      url = "https://raw.githubusercontent.com/milkypostman/melpa/3cfa28c7314fa57fa9a3aaaadf9ef83f8ae541a9/recipes/irony";
+      sha256 = "1xcxrdrs7imi31nxpszgpaywq4ivni75hrdl4zzrf103xslqpl8a";
+      name = "recipe";
+    };
+    preConfigure = ''
+      cd server
+    '';
+    preBuild = ''
+      make
+    '';
+    postInstall = ''
+      mkdir -p $out
+      mv $out/share/emacs/site-lisp/elpa/*/server/bin $out
+      rm -rf $out/share/emacs/site-lisp/*/server
+    '';
+    preCheck = ''
+      cd source/server
+    '';
+    dontUseCmakeBuildDir = true;
+    doCheck = true;
+    packageRequires = [ emacs ];
+    nativeBuildInputs = [ external.cmake external.llvmPackages.llvm ];
+    meta = {
+      homepage = "https://melpa.org/#/irony";
+      license = lib.licenses.gpl3;
+    };
+  };
+
   redshank = callPackage ../applications/editors/emacs-modes/redshank { };
 
   rtags = melpaBuild rec {