Check CMAKE_POLICY_DEFAULT_CMP<NNNN> for a default when policy CMP<NNNN>
would otherwise be left unset.  This allows users to set policies on the
command line when the project does not set them.  One may do this to
quiet warnings or test whether a project will build with new behavior
without modifying code.  There may also be cases when users want to
build an existing project release using new behavior for policies
unknown to the project at the time of the release.