* 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
Daniel Mensinger