(could a MOD perhaps split the GPL discussion a separate topic? ... its a bit OT )
they will have commercial licenses... as originally it wasn't open source at all.
(I know for sure they have JUCE, id have to check FFTW , but Id be extremely surprised/disappointed etc, if they didnt have that too)
do you know how GPL deals with 'optional' modules?
EigenD can be used without a pico, (with a tau/alpha, or midi mpe, osc) , so can you have an optional module which is not compiled into the release... thereby you are not releasing any binary code, for something you cannot compile. (it would be painful for users, but a possible solution)
also, how do open source projects deal with VSTs, since the steinberg api is not open source, you have to register to download it (and therefore agree terms) , and as a derivative product are not allowed to include it in your source base.
(and juce arguably, optionally, relies on the steinberg api, for its VST support)