PocketFFT (3rd/pocketfft, vendored 2018 version) cannot be compiled with
-fno-exceptions, which PIP uses on all non-Windows/non-Android targets.
This adds the proposed upstream patch adding a POCKETFFT_NO_EXCEPTIONS
option (error paths abort with a diagnostic instead of throwing), plus
the PR description for submission to mreineck/pocketfft.
Verified: PIP builds with current upstream master + this patch and
'-fno-exceptions -DPOCKETFFT_NO_EXCEPTIONS -DPOCKETFFT_NO_MULTITHREADING'
and passes the FFT functional test suite.
After the upstream PR is merged, bump the vendored header and drop the
local workarounds.
