Disallow add_custom_command SOURCE signatures.
CMake 2.8.12 and lower allowed a signature for
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.
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 3.19.8 warns when the policy is not set and uses
OLD behavior. Use the
cmake_policy() command to set it to
OLD behavior of a policy is
deprecated by definition
and may be removed in a future version of CMake.