title: How to clear/delete a cache variable
body: <p>Trying to get <code>find_path</code> to do what I want.</p>
  
  <pre><code>find_path(temmp include/help.h)
  message("temmp= ${temmp}")
  </code></pre>
  
  <p>help.h is found.  The output is  <code>temmp= /usr/local/toolA</code></p>
  
  <pre><code>find_path(temmp include/foo.shoe)
  message("temmp= ${temmp}")
  </code></pre>
  
  <p>foo.shoe does not exist (not found).  The output is <code>temmp= /usr/local/toolA</code>
  The cache variable exists, so the variable (temmp) is untouched.</p>
  
  <p>I try and clear the cache var with this:</p>
  
  <pre><code>set (temmp "" CACHE INTERNAL "")
  find_path(temmp include/help.h)
  message("temmp= ${temmp}")
  </code></pre>
  
  <p>No change.  The variable is cleared, but still exists.  The output is <code>temmp=</code>
  (<code>find_path</code> does not run.)</p>
  
  <p>How can I delete the <code>temmp</code> variable from the cache?  (I want to force the <code>find_path</code> to run again.)</p>