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 |release| warns when the policy is not set and uses OLD behavior. Use the cmake_policy command to set it to OLD or NEW explicitly.