Don't refer to ENVIRONMENT_IS_PTHREADS if not building with pthreads enabled.

Author: juj

src/shell.js

@@ -41,7 +41,9 @@ var ENVIRONMENT_IS_WEB = typeof window === 'object';
 // 2) We could be the application main() thread proxied to worker. (with Emscripten -s PROXY_TO_WORKER=1) (ENVIRONMENT_IS_WORKER == true, ENVIRONMENT_IS_PTHREAD == false)
 // 3) We could be an application pthread running in a worker. (ENVIRONMENT_IS_WORKER == true and ENVIRONMENT_IS_PTHREAD == true)
 var ENVIRONMENT_IS_WORKER = typeof importScripts === 'function';
+#if USE_PTHREADS
 if (typeof ENVIRONMENT_IS_PTHREAD === 'undefined') ENVIRONMENT_IS_PTHREAD = false; // ENVIRONMENT_IS_PTHREAD=true will have been preset in pthread-main.js.
+#endif
 var ENVIRONMENT_IS_SHELL = !ENVIRONMENT_IS_WEB && !ENVIRONMENT_IS_NODE && !ENVIRONMENT_IS_WORKER;
 
 if (ENVIRONMENT_IS_NODE) {