CMP0046
-------

Error on non-existent dependency in add_dependencies.

CMake 2.8.12 and lower silently ignored non-existent dependencies
listed in the :command:`add_dependencies` command.

The OLD behavior for this policy is to silently ignore non-existent
dependencies. The NEW behavior for this policy is to report an error
if non-existent dependencies are listed in the :command:`add_dependencies`
command.

This policy was introduced in CMake version 3.0.
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.