Input directories must have CMakeLists.txt.
CMake versions before 2.8 silently ignored missing CMakeLists.txt files in directories referenced by add_subdirectory() or subdirs(), treating them as if present but empty. In CMake 2.8.0 and above this policy determines whether or not the case is an error. The OLD behavior for this policy is to silently ignore the problem. The NEW behavior for this policy is to report an error.
This policy was introduced in CMake version 2.8.0. CMake version 3.9.6 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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