CMP0040

The target in the TARGET signature of add_custom_command() must exist.

CMake 2.8.12 and lower silently ignored a custom command created with the TARGET signature of add_custom_command() if the target is unknown.

The OLD behavior for this policy is to ignore custom commands for unknown targets. The NEW behavior for this policy is to report an error if the target referenced in add_custom_command() is unknown.

This policy was introduced in CMake version 3.0. CMake version 3.1.3 warns when the policy is not set and uses OLD behavior. Use the cmake_policy command to set it to OLD or NEW explicitly.