title: "Eigen3Config.cmake" is not installed in CMake
body: <p>I've installed Eigen3 using CMake, following everything in the guide (<a href="https://gitlab.com/libeigen/eigen/-/blob/master/INSTALL" rel="nofollow noreferrer">https://gitlab.com/libeigen/eigen/-/blob/master/INSTALL</a>). I had tried to CMake Pangolin for my project, but it returned that &quot;Eigen3Config.cmake&quot; not found: <a href="https://i.stack.imgur.com/pdMEE.png" rel="nofollow noreferrer">https://i.stack.imgur.com/pdMEE.png</a>. I've tried to set the path manually only to find out that I don't have the Eigen3Config (nor the eigen3-config).
  Not even Eigen3Config.cmake.in in eigen/cmake. I'd tried to search for the config file but it  returned nothing: <a href="https://i.stack.imgur.com/qxfDF.png" rel="nofollow noreferrer">https://i.stack.imgur.com/qxfDF.png</a></p>
  <p>I have eigen-3.2.10 (older version required for project).</p>