build: Allow building sdist even if FFTW libs are not found

[dalcinl]

No description provided.

[dalcinl]

@mikaem Could you please add me an an OWNER of the PyPI project (https://pypi.org/manage/project/mpi4py-fft/releases/). Afterwards, I'll setup Trusted Publishing https://pypi.org/manage/project/mpi4py-fft/settings/publishing/ such that release tarballs are automatically generated and uploaded after we publish a new GitHub release.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.37%. Comparing base (a69790e) to head (1ee08a0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #42   +/-   ##
=======================================
  Coverage   71.37%   71.37%           
=======================================
  Files          13       13           
  Lines        1324     1324           
=======================================
  Hits          945      945           
  Misses        379      379           

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[mikaem]

@mikaem Could you please add me an an OWNER of the PyPI project (https://pypi.org/manage/project/mpi4py-fft/releases/). Afterwards, I'll setup Trusted Publishing https://pypi.org/manage/project/mpi4py-fft/settings/publishing/ such that release tarballs are automatically generated and uploaded after we publish a new GitHub release.

@dalcinl I've added you as owner.