Disallow add_custom_command SOURCE signatures.
CMake 2.8.12 and lower allowed a signature for
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 than their input.
The OLD behavior for this policy is to allow the use of
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 3.9.0 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.
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