title: CMake does not find the source directory (installing Eigen)
body: <p>I was installing the library <a href="http://eigen.tuxfamily.org/index.php" rel="nofollow">Eigen</a> on Ubuntu. I followed the instructions by creating a "build directory" alongside the downloaded source directory. That is, I have two directories: <code>eigen-eigen-1306d75b4a21</code> for source, and an empty <code>eigen-build</code> for build.</p>
  
  <p>Then I did:</p>
  
  <pre><code>cd eigen-build
  cmake ../eigen-eigen-1306d75b4a21/
  sudo make install
  </code></pre>
  
  <p>An error occurred immediately after <code>make install</code> and it stops the installation:</p>
  
  <pre><code>CMake Error: The source directory "/home/username/something/eigen-eigen-1306d75b4a21" does not exist.
  </code></pre>
  
  <p>Obviously the directory exists. What is wrong with my CMake? (I don't know much about CMake. I observed some warnings during the second step, saying lots of things such as Qt4 are missing, but I don't think this is the cause, because of what the error message suggests.)</p>