Fix with-styled-jsx-postcss example (#1147)

examples/with-styled-jsx-postcss/.babelrc

 {
   "presets": [
-    "next/babel"
-  ],
-  "plugins": [
-    "styled-jsx-postcss/babel"
+    "./babel-preset"
   ]
 }

examples/with-styled-jsx-postcss/README.md

@@ -3,6 +3,9 @@
 
 This example features how you use PostCSS with styled-jsx via [styled-jsx-postcss](https://github.com/giuseppeg/styled-jsx-postcss)
 
+N.B. In order to integrate `styled-jsx-postcss` with Next.js you need to patch Next.js'
+babel preset. See [babel-preset.js](./babel-preset.js) and [.babelrc](./.babelrc).
+
 ## How to use
 
 Download the example [or clone the repo](https://github.com/zeit/next.js):

examples/with-styled-jsx-postcss/babel-preset.js

+const nextBabelPreset = require('next/babel')
+
+nextBabelPreset.plugins = nextBabelPreset.plugins.map(plugin => {
+  if (!Array.isArray(plugin) && plugin.indexOf('styled-jsx/babel') !== -1) {
+    return require.resolve('styled-jsx-postcss/babel')
+  }
+  return plugin
+})
+
+module.exports = nextBabelPreset