Visual Studio Windows Target Platform Version.

When targeting Windows 10 and above Visual Studio 2015 and above support specification of a target Windows version to select a corresponding SDK. The CMAKE_SYSTEM_VERSION variable may be set to specify a version. Otherwise CMake computes a default version based on the Windows SDK versions available. The chosen Windows target version number is provided in CMAKE_VS_WINDOWS_TARGET_PLATFORM_VERSION. If no Windows 10 SDK is available this value will be empty.