Allow users to specify defaults for unset policies
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.