title: predefined macro for 64 bit systems
body: <p>I have to write a function which searches different directories for different (32/64 bit) machine. 
  For example :</p>
  
  <pre><code> //if 64 bit
   Opts.AddPath("/usr/include/x86_64-linux-gnu");
  
   //if 32 bit
   Opts.AddPath("/usr/include/i386-linux-gnu");
  </code></pre>
  
  <p>Is there a predefined/compiler defined macro that lets me select a particular code.</p>
  
  <p>I am using gcc on Ubuntu machine.</p>
  
  <p>Thanks.</p>