lib: deep-copy process.config during configure (#2368)

Author: DeeDeeG

lib/configure.js

@@ -98,7 +98,7 @@ function configure (gyp, argv, callback) {
 
     log.verbose('build/' + configFilename, 'creating config file')
 
-    var config = Object.assign({}, process.config)
+    var config = process.config ? JSON.parse(JSON.stringify(process.config)) : {}
     var defaults = config.target_defaults
     var variables = config.variables