Adding Optional Support for pthreads

On one of my projects, I decided I wanted to provide optional support for pthreads, because I wanted portability to dinosaur compilers. Not only did I want pthreads, but I only wanted to use them if <stdatomic.h> was available. This took a bit of finagling to get right, so I’ve written it up here. Per the Autotools Mythbuster , the correct way to indicate optional dependencies is to check for them…