VS: Fix standalone Windows7.1SDK toolset selection

Since commit v3.7.0-rc1~142^2~1 (VS: Explicitly default to v100 toolset
in Visual Studio 2010, 2016-07-21) we prefer to always set a platform
toolset explicitly so that `CMAKE_VS_PLATFORM_TOOLSET` can be reported.
However, the `v100` default for the VS 10 generator is not appropriate
for all environments.  We fixed support for VS 2010 Express Edition in
commit v3.7.0-rc1~142^2 (VS: Do not default to missing v100 64-bit
toolset on VS 2010 Express, 2016-09-09).

Fix support for the standalone Windows7.1SDK toolset environment by
recognizing the `PlatformToolset` environment variable that it sets to
`Windows7.1SDK` and using this as our default toolset instead.

Closes: #16483

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