Use the FindEigen3.cmake from the Eigen repository

Downloaded from https://bitbucket.org/eigen/eigen/src/0c883fcd96da1bcfc697bb06a421ad567689a8b2/cmake/FindEigen3.cmake .
The used  `FindPackageHandleStandardArgs` CMake module is available since CMake 2.6 , that is already the minimum CMake version required by orocos_kdl. 

The old `FindEigen3.cmake` script is not compatible with Windows, this one should work with Windows and also cover the old use cases.