[boost] GPL and boost

Hi, is there a precedent of optional(!) external library dependencies to GPLed libraries in boost? IIUC the implementation of the interface to the library in boost would constitute a derivative work of the GPLed library and therefore be "infected" by it. do the terms of the GPL then only apply if a boost user is actually using this optional code and linking to the external library or does this inhibit the distribution of such code with boost releases in general? and if so, is external distribution, e.g. in the vault, but documentation in the main library acceptable? I'm asking because of a possible interface to Oracle's Berkeley DB, but I guess this will come up again with Boost.Rdb and GPLed SQL databases.