CMakeBackwardCompatibilityCXX

определяет набор переменных обратной совместимости

CMAKE_ANSI_CXXFLAGS - flag for ansi c++
CMAKE_HAS_ANSI_STRING_STREAM - has <strstream>
include(TestForANSIStreamHeaders)
include(CheckIncludeFileCXX)
include(TestForSTDNamespace)
include(TestForANSIForScope)

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https://cmake.org/cmake/help/v3.20/module/CMakeBackwardCompatibilityCXX.html