[boost] `b2 toolset=clang-win` now works on Appveyor

With the current develop branch (since Boost.Build hasn't been merged to master yet), b2 toolset=clang-win now works on Appveyor without additional configuration. You can see it in action at https://ci.appveyor.com/project/pdimov/assert/builds/20331077/job/0hux9a6mlr...