require fftw 3.3.9 and use planner_nthreads in finufft.cpp

https://github.com/flatironinstitute/finufft/blob/cdcccbec2585ae2ab5bf2e0be27bac1c526fa6c3/src/finufft.cpp#L764

rewrite that fftw plan call using fftw_planner_nthreads.
See https://www.fftw.org/release-notes.html
3.3.9 was released Dec 2020.

Julia crash may be related to this:
https://github.com/ludvigak/FINUFFT.jl/issues/61