CMAKE_VS_WINRT_BY_DEFAULT

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.