It is likely that projects or CMake modules in the future will need to
check the value of a policy setting. For example, if we add a policy
that affects the results of FindXYZ.cmake modules, the module code will
need to be able to check the policy.
- In cmake_minimum_required do not set policy version if current
CMake is too old
- In cmPolicies::ApplyPolicyVersion report error if version is too
new or cannot be parsed