Further fixes for CMAKE_INSTALL_PREFIX correctness

And other related cmake cleanup, including:

- Use CMAKE_CURRENT_LIST_DIR to find UseEigen3.cmake
- Use INSTALL_DIR term consistently for variable names
- Drop unnecessary extra EIGEN_INCLUDE_INSTALL_DIR
- Fix some paths in generated eigen3.pc and Eigen3Config.cmake files
    missing CMAKE_INSTALL_PREFIX
- Fix pkgconfig directory choice ignored if it doesn't exist at configure
    time (bug #711)