Fix for default NODE_JS in config file (#369)

This was broken in #362

emsdk.py

@@ -1351,7 +1351,7 @@ def generate_dot_emscripten(active_tools):
     node_fallback = which('nodejs')
     if not node_fallback:
       node_fallback = 'node'
-    activated_config['NODE_JS'] = node_fallback
+    activated_config['NODE_JS'] = "'%s'" % node_fallback
 
   for name, value in activated_config.items():
     cfg += name + ' = ' + value + '\n'