| Disallow add_custom_command SOURCE signatures. |
| CMake 2.8.12 and lower allowed a signature for :command:`add_custom_command` |
| which specified an input to a command. This was undocumented behavior. |
| Modern use of CMake associates custom commands with their output, rather |
| The OLD behavior for this policy is to allow the use of |
| :command:`add_custom_command` SOURCE signatures. The NEW behavior for this |
| policy is to issue an error if such a signature is used. |
| 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 |
