In the CMake find module, define the Eigen imported target as when installing with CMake
* In the CMake find module, define the Eigen imported target
* Add quotes to the imported location, in case there are spaces in the path.
Approved-by: Alain Vaucher <acvaucher@gmail.com>
* add a file Eigen/signature_of_eigen3_matrix_library, use it to make FindEigen3.cmake more solid: able to find Eigen in either eigen3/ or eigen/ and not mix it up with Eigen2.