Consider not building CMake on distros with a newer version available

[SwooshyCueb]

As we move towards find_package for more and more dependencies, the complexity that would be introduced by implementing distro-specific packaging in our current externals build system (and the amount of work required to implement it) is reduced. From where I stand today, I'm still uncertain whether it will ever be worth the effort to implement this for most packages, given that we will eventually be moving away from this system anyway.

However, I believe CMake is an exception. At present, switching to distro-provided CMake for distros with a newer version available than what we provide would involve making significantly fewer cross-repository changes than any other package. Moreover, we already use distro-provided CMake for newer distros in a number of places. Let's consider omitting CMake from externals in these cases.

[trel]

Yes - this makes a lot of sense. We have not leaned on a new CMake feature/bugfix in quite some time.