GitLab

* Replace pthreads with C++ threads

* Try to fix CI errors

* Try including -pthread linker option

* Use PocketFFT instead of FFTW

* Minor cleanup

* Use PocketFFT instead of fftpack for reference platform

* Remove FFTW as a dependency

* Converted a test case to use PocketFFT

* Fixed an incorrect comment

Removed AVX code, since it had very little effect on performance and would have required a more complicated build process.  Also worked around a compilation error with clang.