DO NOT CHECK OUT THESE FILES FROM GITHUB UNLESS YOU KNOW WHAT YOU ARE DOING. (See below.)
This is the git repository for the FFTW library for computing Fourier transforms (version 3.x), maintained by the FFTW authors.
Unlike most other programs, most of the FFTW source code (in C) is generated automatically. This repository contains the generator and it does not contain the generated code. You cannot compile code from this repository unless you have special tools and know what you are doing.
Most users should ignore this repository, and should instead download official tarballs from http://fftw.org/, which contain the generated code, do not require any special tools or knowledge, and can be compiled on any system with a C compiler.
Advanced users and FFTW maintainers can obtain code from github and run the generation process themselves. This is a long process that requires special tools. See README for details.