(optionally) use fftw instead of mixfft

[umlaeute]

would it be possible to switch mixfft with fftw?

as it is, I have a pd-vasp package for Debian almost already, but the mixfft license makes it undistributable (for Debian)

[grrrr]

That would certainly be possible and from today's perspective it would be the better choice for non-commercial use. As i see it, FFTW supports all the necessary parts and pieces, although the normalization could be different and should be adapted accordingly. I don't know if i get to that anytime soon, a pull request would be much appreciated.