Set new URL for FFTW download

Summary

The download link for FFTW changed.

Rationale

The SuperBuild uses the URL http://www.fftw.org/ to download FFTW. This URL is not available anymore. The new URL is http://fftw.org/.

Implementation Details

Update the URLs in SuperBuild/CMake/External_fftw.cmake

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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Check before merging:

• All discussions are resolved
• At least 2 votes from core developers, no vote.
• The feature branch is (reasonably) up-to-date with the base branch
• Dashboard is green
• Copyright owner has signed the ORFEO ToolBox Contributor License Agreement
• Optionally, run git diff develop... -U0 --no-color | clang-format-diff.py -p1 -i on latest changes and commit