Avoid FFTW dependency because of GPL

[andreasnoack]

The dependency on FFTW effectively makes this package copyleft, which makes it harder to use in some applications. Eventually, it might be possible to use either https://github.com/Taaitaaiger/RustFFT.jl or https://github.com/JuliaMath/FFTA.jl, although they currently lack some functionality, so it might not be easy to avoid the dependency short term.

[wheeheee]

AFAIU if the end user has to download FFTW it's likely to be fine (JuliaMath/FFTW.jl#41 (comment)). If that still doesn't fit your use case, would it be easier to use MKL with FFTW.jl (bindings are MIT-licensed)?

[andreasnoack]

Unfortunately, it is not always possible to work around the cnn license restrictions like that. E.g. it might be necessary to distribute a complete bundle and sometimes you'd have to distribute for architectures not supported by MKL.

[wheeheee]

I guess you would have to fork this repo for niche platforms. I doubt DSP.jl (and most other packages in the Julia ecosystem that use FFTs) will ever switch away from FFTW, unless in the near future there's an FFTW drop-in with a more permissive license. That's not going to come anytime soon though.