Improve cpp nix builder

nix/squoosh-cxx-builder/default.nix

@@ -0,0 +1,31 @@
+{
+  pkgs,
+  stdenv,
+  runCommand,
+}:
+{
+  name,
+  src,
+  nativeBuildInputs ? [ ],
+  encoder ? "enc",
+  decoder ? "dec",
+  ...
+}@args:
+
+stdenv.mkDerivation (
+  final:
+  args
+  // {
+    inherit name src;
+    nativeBuildInputs = [ pkgs.emscripten ] ++ nativeBuildInputs;
+    buildPhase = ''
+      export HOME=$TMPDIR
+      emmake make -j$(nproc)
+    '';
+    installPhase = ''
+      mkdir -p $out
+      ${if (encoder != null) then "cp -r ${encoder} $out" else ""}
+      ${if (decoder != null) then "cp -r ${decoder} $out" else ""}
+    '';
+  }
+)