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 :command:`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 :command:`add_custom_command` is unknown.

This policy was introduced in CMake version 3.0.0.  CMake version
|release| warns when the policy is not set and uses OLD behavior.  Use
the cmake_policy command to set it to OLD or NEW explicitly.
