Fix Javascript examples so that "make clean" works properly with node

- Need to copy example.cxx to build directory so that build products
  end up in the right place; use a gyp command expansion to do so

Examples/javascript/class/binding.gyp.in

@@ -2,7 +2,7 @@
   "targets": [
     {
       "target_name": "example",
-      "sources": [ "$srcdir/example.cxx", "example_wrap.cxx" ],
+      "sources": [ "<!(cp $srcdir/example.cxx example-gypcopy.cxx && echo example-gypcopy.cxx)", "example_wrap.cxx" ],
       "include_dirs": ["$srcdir"]
     }
   ]