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.
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