title: declare a constant string with a condition
body: <p>i have a source file to put all my constants ( <code>constant.h</code>) like this :</p>
  
  <pre><code>#define MY_URL @"url"
  #define SECOND_URL @"url2"
  ...
  </code></pre>
  
  <p>my problèm is to déclare a constant with a condition like this :</p>
  
  <pre><code>if (ipad)
     #define MY_CONSTANT @"ipad"
  else
     #define MY_CONSTANT @"iphone"
  </code></pre>
  
  <p>How i can do this please and put it in the <code>constant.h</code> ?</p>