Cleanup and reorganise CMake config

This branch updates the repository's CMake configuration files to follow modern best practices, e.g. as outlined in the official CMake tutorial as well as https://cliutils.gitlab.io/modern-cmake/chapters/install/installing.html and https://pabloariasal.github.io/2018/02/19/its-time-to-do-cmake-right/ (among other pages). It's a partner to herd-library!12 (merged)

Note for future: In the end, I reverted an update of the recommended build/install commands from make / make install to cmake3 --build . / cmake3 --install ., as with those commands installation would fail in CentOS7 arm64 images. This problem may have occurred because the ARM64 image contained CMake 3.14.6, whereas the x86 one contained version 3.17 (in both cases CMake was installed via YUM, using EPEL repos).