I changed the project-config.jam file in my Boost source tree to have Boost.Build use LLVM, but then when I used "bootstrap" command after, I saw that it was still using MSVC.  When I looked at the project-config.jam file again at that point, I saw that its content had been reverted back to the default one for building with MSVC.  What's going on?  What do I need to do?  Is there an argument I need to provide to the bootstrap command here so that it'll keep the configuration file content that I want it to?  And is there no help command that would let me see what arguments, if any, I can pass to the tool?

Another thing I want to ask is whether or not I need to have Boost.Built emit library files with a .bc extension to be able to link them with code I want to build using the Emscripten compiler.  I ask because Emscripten needs compiled libraries to have been compiled to LLVM bitcode, and when Emscripten itself does this it adds the .bc extension instead of the .lib one.