VS: Tell VS 2019 to use Windows SDK 8.1 explicitly when needed
VS 2019 does not default to the 8.1 SDK as VS 2017 and VS 2015 did. When CMAKE_SYSTEM_VERSION is 8.1 or lower, select the 8.1 SDK explicitly.
Fixes: #18927 (closed)
VS 2019 does not default to the 8.1 SDK as VS 2017 and VS 2015 did. When CMAKE_SYSTEM_VERSION is 8.1 or lower, select the 8.1 SDK explicitly.
Fixes: #18927 (closed)