Update usage strings if --gypjs command-line option was supplied

Set `npm_config_gypjs` env variable to affect usage text for `configure`
and `build` commands.

Author: pmed

lib/node-gyp.js

@@ -176,6 +176,9 @@ proto.parseArgv = function parseOpts (argv) {
   if (this.opts.loglevel) {
     log.level = this.opts.loglevel
   }
+  if (this.opts.gypjs) {
+    process.env[npm_config_prefix + 'gypjs'] = true
+  }
   log.resume()
 }