Assume I have a library A which is licensed under a two-clause Free BSD style license. Library A optionally depends on library B (the availability of the dependency is configured at build-time), which is licensed under the GPLv3.

If I distribute both bundled together, the license will need to be GPL. But am I still able to distribute library A under the FreeBSD license? How do I indicate that the license changes, when the use of library B is enabled?

Do I need to distribute two different versions or can I just have one that contains both licenses and states which applies under which conditions? Any example project I can have a look at to see it done?