Show how to use the [Eigen library](http://eigen.tuxfamily.org) with [OpenMP](https://www.openmp.org/) threading and optionally linking
to BLAS/LAPACK.
Usage of CMake [`IMPORTED` libraries](https://cmake.org/cmake/help/latest/manual/cmake-buildsystem.7.html#imported-targets) is shown for CMake 3.9 or higher.
The `cxx-example-3.5` folder shows how to attain the same goal with older
versions of CMake.
