_DEBUG should not be defined in Windows-MSVC.cmake
CMake should not be defining /D_DEBUG explicitly in CMAKE_C/CXX_FLAGS_DEBUG_INIT, as it is used by Microsoft’s headers to match the chosen runtime (/MDd or /MTd).
I'm not sure if the init for Clang should also be updated to remove -D_DEBUG.
Related email thread https://cmake.org/pipermail/cmake/2016-November/064487.html
Related line: https://github.com/Kitware/CMake/blob/b53a12c29cff61d8f97997a63a00e7ca59b566ba/Modules/Platform/Windows-MSVC.cmake#L294