FFTW should be a dependency

[leofang]

I think after #7 for pip install to work we need the presence of FFTW. We'll need a mechanism to detect FFTW headers at build time (similar to how pybind11 is handled). For conda recipe it's easy (just set fftw to be a build dependency), but I'm not sure how to cleanly resolve this for PyPI packaging...

[tacaswell]

There is a setup_requires field that mostly works. You can also go down the pyproject.toml / flit route.

[leofang]

If FFTW is not found, pip install from PyPI should just fail (as it does now), right?