Update to FFTW 3.3.4

Upstream update, looks pretty minor: https://github.com/FFTW/fftw3/blob/master/NEWS
2 of the 6 changes are solving problems on Windows that we had local Julia patches to fix, which we can now get rid of. Any reason not to do this now @stevengj ?

I can add checksum files if #6773 gets merged first.