Ancient versions of CMake required else(), endif(), and similar block
termination commands to have arguments matching the command starting the block.
This is no longer the preferred style.
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.
