I was talking about LLVM/Clang on Windows.  I want to have compiled binaries for the Boost libraries generated by both the MSVC and Clang compilers.  I'm building LLVM version 8 from source and am also going to build Boost using MSVC first, with the default project-config.jam file.  [The reason I'm building version 8 is so that I can use it to target WebAssembly as well.]