title: Syntaxerror 'varying' in GLSL
body: <p>I'm using GLFW 3 and OpenGL 4 + GLSL 4 on a MacBook Pro.
  I get the following syntax error, when starting my program:</p>
  
  <pre><code>ERROR: 0:5: 'varying' : syntax error syntax error
  </code></pre>
  
  <p>The shader code:</p>
  
  <pre><code>#version 410
  
  varying vec3 vertex;
  
  void main() {
  }
  </code></pre>
  
  <p>Why am I not allowed to use varying variables?</p>