Input directories must have
CMake versions before 2.8 silently ignored missing
files in directories referenced by
treating them as if present but empty. In CMake 2.8.0 and above this
cmake_policy() determines whether or not the case is an error.
OLD behavior for this policy is to silently ignore the problem.
NEW behavior for this policy is to report an error.
This policy was introduced in CMake version 2.8.0. CMake version
3.20.20210512-g35720ce warns when the policy is not set and uses
OLD behavior. Use
cmake_policy() command to set it to
OLD behavior of a policy is
deprecated by definition
and may be removed in a future version of CMake.