lib: avoid changing process.config

It is deprecated and will emit a warning in Node.js 16.

Refs: nodejs/node#36902

Author: targos

lib/configure.js

@@ -96,7 +96,7 @@ function configure (gyp, argv, callback) {
 
     log.verbose('build/' + configFilename, 'creating config file')
 
-    var config = process.config || {}
+    var config = Object.assign({}, process.config)
     var defaults = config.target_defaults
     var variables = config.variables