Eigen assumes that CMAKE_INSTALL_INCLUDEDIR is a path relative to
CMAKE_INSTALL_PREFIX (typically "include"), but CMake supports it being an
absolute path, which is the case in Nixpkgs. This resulted in EIGEN3_INCLUDE_DIR
being set to "/nix/store/…eigen…//nix/store/…eigen…/include/eigen3".
GNUInstallDirs_get_absolute_install_dir requires CMake 3.7.
