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.