title: GCC how to force redefinition warning to error
body: <p>In gcc I can pass option </p>
  
  <pre><code>-Werror=
  </code></pre>
  
  <p>to force a particular warning as error. But I am not able to find option I need to pass for warning  </p>
  
  <pre><code>redefined [enabled by default]
  </code></pre>
  
  <p>to an error. 
  What option should I pass so if a user defined macro is redefined it force it to an error. </p>