VS: force Win10 SDK be chosen for VS2017
Visual Studio 2017 currently only have Win10 SDK bundled while would still switch to choose Win8.1 SDK without
<WindowsTargetPlatformVersion> property set in Visual Studip project file. Therefore, the fact that CMake used to choose correct Win10 SDK only on Windows 10 and later even for Visual Studio 2017 Generator leads to the result that Visual Studio 2017 would report "Could not locate Win8.1 SDK" error when building targets on systems such as Window 7, especially for cases in 'try_run' command where CMake variables seems not to work there.
This patch is intended to fix the problem described above.