VS: Fix standalone Windows7.1SDK toolset selection
Since commit 059c230d 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 861f1b3d.
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 (closed)