VS: Do not fail on Windows 10 with VS 2015 if no SDK is available (#15929)
Since commit v3.4.0-rc1~5^2~1 (VS: Add support for selecting the Windows 10 SDK, 2015-09-30) the VS 2015 generator requires a Windows 10 SDK to be available when CMAKE_SYSTEM_VERSION specifies Windows 10 (e.g. when building on a Windows 10 host). Howewver, it is possible to install VS 2015 without any Windows 10 SDK. Instead of failing with an error message about the lack of a Windows 10 SDK, simply tolerate this case and use the default Windows 8.1 SDK. Since building for Windows Store still requires the SDK, retain the diagnostic in that case.