Support for ARM based architectures

[sindre-nistad]

gaussianfft should work with Apple Silicon, and ARM based Linux.

There are a couple of options;

• Use fftw3 (GPL v2 or later)
• Use <Accelerate>'s vDSP (Apple only)
• ARM Performance Library
• pffft?
• CMSIS/DSP
• There may be other (good) alternatives

#26 originally used fftw3, but I removed it from that PR due to concerns with GPL license.
If it is to be included or used, it would be separate from that PR.