cmake: Require cmake 3.16 or later.

[waywardmonkeys]

Support for requiring cmake < 3.4 may go away soon (according to a deprecation notice when building).

Ubuntu 20.04 provides cmake 3.16 and current is 3.27, so that seems like a reasonable version to require.

[waywardmonkeys]

I need this for another change that will requires cmake 3.12 or later, but wanted to introduce this separately to ensure no issues with it.

[aytey]

Can I suggest 3.12 rather than 3.16 then? It seems like this would exclude some users “pointlessly” if you only need 3.12 features.

[waywardmonkeys]

Well, I don’t think this is actually tested, so not really sure that it even works on any given version. Is there someone that is actually stuck on something as old as 3.12?

[waywardmonkeys]

FWIW, 3.12 was released in July 2018 and 3.16 in November 2020.

Along similar lines, Z3 requires C++17 and suggests or needs at least Visual Studio 2019 on Windows.

[NikolajBjorner]

The way I see it: It is a non-goal for z3 builds to be compatible with stale software pipelines, so 3.16 (instead of 3.12) makes better sense. Downlevel compatibility is a well-known ask for users not upgrading their pipelines, but it is a trap when it comes to flexiblity.