Fix: Basic webpack 4 support (styleguidist#849)

To have webpack 4 configured correctly we need to set the webpack mode.

Author: KnisterPeter

scripts/make-webpack-config.js

@@ -10,6 +10,7 @@ const merge = require('webpack-merge');
 const forEach = require('lodash/forEach');
 const mergeWebpackConfig = require('./utils/mergeWebpackConfig');
 const StyleguidistOptionsPlugin = require('./utils/StyleguidistOptionsPlugin');
+const getWebpackVersion = require('./utils/getWebpackVersion');
 
 const RENDERER_REGEXP = /Renderer$/;
 
@@ -51,6 +52,10 @@ module.exports = function(config, env) {
 		},
 	};
 
+	if (getWebpackVersion() >= 4) {
+		webpackConfig.mode = env;
+	}
+
 	if (isProd) {
 		webpackConfig = merge(webpackConfig, {
 			output: {