[CMake] How to maintain compatibility with earlier versions?

[Robert Dailey]

So typically my setup is that I have the newest CMake installed, but
am working with projects that set cmake_minimum_required to something
like version 2.8. Will version 3.9 of CMake prevent me from using
features that were introduced after 2.8? How can I make sure that
someone with *actual* version 2.8 installed will not run into
errors/issues?