title: ctest does not find valgrind
body: <p>Calling</p>
  
  <pre><code>ctest -j4 -DCTEST_MEMORYCHECK_COMMAND="/usr/bin/valgrind" -DMemoryCheckCommand="/usr/bin/valgrind" --output-on-failure -T MemCheck 
  </code></pre>
  
  <p>says</p>
  
  <pre><code>Memory checker (MemoryCheckCommand) not set, or cannot find the specified program.
  </code></pre>
  
  <p>Why doesn't it find valgrind automatically nor when specified manually?</p>