Issue a deprecation warning on calls to `cmake_minimum_required` or
`cmake_policy` that set policies based on versions older than 3.5.
Note that the effective policy version includes `...<max>` treatment.
Update the check from commit 5845c218d7 (Deprecate compatibility with
CMake versions older than 2.8.12, 2020-06-12, v3.19.0-rc1~629^2).
When CMP0053 is in WARN mode, variables get expanded twice, leaking the
fact that the string was expanded twice and changing behavior. Instead,
suppress variable watches when running the expansion to trigger the
CMP0053 warning.
Brad King