emacsPackages.vterm: fix dynamic library and autoloaders

We need both the .so as well as the proper autoloaders that are generated when
we build an emacs package, so we cannot use the normal stdenv.mkDerivation but
have to inject the building of the libraries instead.

Also, use the proper libvterm-neovim we have in nixpkgs instead of vendoring it.

pkgs/applications/editors/emacs-modes/melpa-packages.nix

@@ -385,52 +385,22 @@ env NIXPKGS_ALLOW_BROKEN=1 nix-instantiate --show-trace ../../../../ -A emacsPac
             (attrs.nativeBuildInputs or []) ++ [ external.git ];
         });
 
-        vterm = let
-          emacsSources = pkgs.stdenv.mkDerivation {
-            name = self.emacs.name + "-sources";
-            src = self.emacs.src;
-
-            dontConfigure = true;
-            dontBuild = true;
-            doCheck = false;
-            fixupPhase = ":";
-
-            installPhase = ''
-              mkdir -p $out
-              cp -a * $out
-            '';
-
-          };
-
-          libvterm = pkgs.libvterm-neovim.overrideAttrs(old: rec {
-            pname = "libvterm-neovim";
-            version = "2019-04-27";
-            name = pname + "-" + version;
-            src = pkgs.fetchFromGitHub {
-              owner = "neovim";
-              repo = "libvterm";
-              rev = "89675ffdda615ffc3f29d1c47a933f4f44183364";
-              sha256 = "0l9ixbj516vl41v78fi302ws655xawl7s94gmx1kb3fmfgamqisy";
-            };
-          });
-
-        in pkgs.stdenv.mkDerivation {
-          inherit (super.vterm) name version src;
-
-          nativeBuildInputs = [ pkgs.cmake ];
-          buildInputs = [ self.emacs libvterm ];
-
+        vterm = super.vterm.overrideAttrs(old: {
+          buildInputs = old.buildInputs ++ [ self.emacs pkgs.cmake pkgs.libvterm-neovim ];
           cmakeFlags = [
-            "-DEMACS_SOURCE=${emacsSources}"
-            "-DUSE_SYSTEM_LIBVTERM=True"
+            "-DEMACS_SOURCE=${self.emacs.src}"
+            "-DUSE_SYSTEM_LIBVTERM=ON"
           ];
-
-          installPhase = ''
-            install -d $out/share/emacs/site-lisp
-            install ../*.el $out/share/emacs/site-lisp
-            install ../*.so $out/share/emacs/site-lisp
+          # we need the proper out directory to exist, so we do this in the
+          # postInstall instead of postBuild
+          postInstall = ''
+            pushd source/build >/dev/null
+            make
+            install -m444 -t $out/share/emacs/site-lisp/elpa/vterm-** ../*.so
+            popd > /dev/null
+            rm -rf $out/share/emacs/site-lisp/elpa/vterm-**/{CMake*,build,*.c,*.h}
           '';
-        };
+        });
         # Legacy alias
         emacs-libvterm = unstable.vterm;