CMAKE_VS_WINRT_BY_DEFAULT

New in version 3.13.

Inform Visual Studio Generators for VS 2010 and above that the target platform enables WinRT compilation by default and it needs to be explicitly disabled if /ZW or VS_WINRT_COMPONENT is omitted (as opposed to enabling it when either of those options is present)

This makes cmake configuration consistent in terms of WinRT among platforms - if you did not enable the WinRT compilation explicitly, it will be disabled (by either not enabling it or explicitly disabling it)

Note: WinRT compilation is always explicitly disabled for C language source files, even if it is expliclty enabled for a project

This variable is meant to be set by a toolchain file for such platforms.