* backends/vs: Only set platform_toolset if it isn't already set
* interpreter: set backend up after the compiler
Otherwise we won't be able to check which VS toolchain to use.
* docs/using-visual-studio: wrap lines
* docs: recommend the py launcher instead of python3 for windows
* set backend.environment when building a dummy version
* backends/vs: Add support for clang-cl with vs2017 and vs2019 backends
* backends/vs: Add support for ICL (19.x) with vs2015 and vs2017 backends
VS2017 requires the 'WindowsTargetPlatformVersion' property to be set.
We gather the version to use from the environment variable
'WindowsSDKVersion' that will be set by the VS developer command prompt.
Dylan Baker