You can have a look here for external libraries. Unfortunately, library names are sometimes platform specifc as well (.a/.lib) so I
As for compiler flags, if it is not part of the built-in features, you would have to add support yourself for each toolset, which is probably even more work.
I had to maintain bjam for a build system at work and I must say, if something does not come in-the-box with it, it can be really hard to find help elsewhere.