title: a macro substitution involving concatenation and line number
body: <p>I would like to have a macro that produce something like <code>L17</code>, where <code>17</code> is the line number when the macro is invoked.  However the following only produce <code>L__LINE__</code></p>
  
  <pre><code>#define STOP L##__LINE__
  </code></pre>
  
  <p>Wonder if there is a way to make the <code>__LINE__</code> evaluate before concatenation.   </p>