output_required_files() command should not be called.
This command was added in June 2001 to expose the then-current CMake implicit dependency scanner. CMake’s real implicit dependency scanner has evolved since then but is not exposed through this command. The scanning capabilities of this command are very limited and this functionality is better achieved through dedicated outside tools.
CMake >= 3.0 prefer that this command never be called. The OLD behavior for this policy is to allow the command to be called. The NEW behavior for this policy is to issue a FATAL_ERROR when the command is called.
This policy was introduced in CMake version 3.0. CMake version 3.7.1 warns when the policy is not set and uses OLD behavior. Use the cmake_policy command to set it to OLD or NEW explicitly.
OLD behavior of a policy is
deprecated by definition
and may be removed in a future version of CMake.