title: How to check the version number of Eigen C++ template library?
body: <p>I added several different versions of Eigen to default including directory of Visual C++.
  But I got collapse problem when using <code>LDLT</code> (Cholesky decomposition) for some of the testing numerical examples.</p>
  
  <p>So I want to determine which version is actually active when debugging the code.</p>
  
  <p>Is there any function which can indicate the current active Eigen version number?</p>