Update to modern CMake

The CMake build system needs to be updated to better reflect the principles used nowadays in the CMake camp.

It is all about targets and properties - and hardly about variables.

https://gist.github.com/mbinna/c61dbb39bca0e4fb7d1f73b0d66a4fd1
and
https://cliutils.gitlab.io/modern-cmake/modern-cmake.pdf

This milestone is worked on in the cmake branch.