Minor cmake tweaks for Pangolin and Eigen

[rAum]

There are 2 changes I think should be included:

1. CMake generates error if Pangolin is not present due to missing ~FindPangolin.cmake file. Pangolin is not a required dependency so find_package should have QUIET.
2. Improved Eigen thirdparty:
   a) Eigen3 is now a target, not set of variables. This means it's now first-class citizen in terms of cmake dependencies.
   b) this requires to have workaround for some unfortunate mistake on Eigen's part that got deployed on some distributions (Ubuntu for example) - Eigen exported vars but not a library.
   This workaround allows to write code as expected from official Eigen doc: https://eigen.tuxfamily.org/dox-devel/TopicCMakeGuide.html

[kzampog]

Awesome, thanks! :D