Is .bc extension required in LLVM bitcode file?

When building Boost using LLVM, do I have to somehow have it compile library files that end with a .bc extension, or is .lib extension fine? I need someone familiar with Emscripten to answer this, since I want to do it for Emscripten.

To add: I changed the project-config.jam file in my Boost source tree to have Boost.Build use LLVM, but then when I use “bootstrap” command, I see that it still uses 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?