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

Subject: [boost] `b2 toolset=clang-win` now works on Appveyor
From: Peter Dimov (lists_at_[hidden])
Date: 2018-11-15 17:09:25

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/0hux9a6mlr4iqvjr