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.
Teach the VS 2015 generator to produce a WindowsTargetPlatformVersion
value. Use the CMAKE_SYSTEM_VERSION to specify the version and if not
set choose a default based on available SDKs. Activate this behavior
when targeting Windows 10.
Co-Author: Brad King <brad.king@kitware.com>
