The GIMPS (Mersenne numbers search) project has a really extremely fast AVX2 assembler implementation of double FFT http://www.mersenne.org/download/ .
So, what it's licensed under? Can I use it in other project? From quick glance, it seems that they have their own license.
The terms and conditions are right here, you can evaluate it for yourself (you are the only person who knows if it suits your needs or not): http://www.mersenne.org/legal/#TCU