Don't restrict CMAKE_BUILD_TYPE This prevents projects that add Eigen using `add_subdirectory` from using their own custom CMAKE_BUILD_TYPE and have Eigen respect the same custom flags.
Eigen is a C++ template library for linear algebra: matrices, vectors, numerical solvers, and related algorithms.
For more information go to http://eigen.tuxfamily.org/.
For pull request, bug reports, and feature requests, go to https://gitlab.com/libeigen/eigen.