CMAKE_BACKWARDS_COMPATIBILITY should no longer be used.
The OLD behavior is to check CMAKE_BACKWARDS_COMPATIBILITY and present it to the user. The NEW behavior is to ignore CMAKE_BACKWARDS_COMPATIBILITY completely.
In CMake 2.4 and below the variable CMAKE_BACKWARDS_COMPATIBILITY was used to request compatibility with earlier versions of CMake. In CMake 2.6 and above all compatibility issues are handled by policies and the cmake_policy command. However, CMake must still check CMAKE_BACKWARDS_COMPATIBILITY for projects written for CMake 2.4 and below.
This policy was introduced in CMake version 2.6.0. 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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