This is done because the library is necessary to make certain FFTW
functions thread safe, see #136557 as well.
Also pass each library variable separately to
`find_package_handle_standard_args` instead of as a list, as otherwise
it won't correctly detect if `libfftw3f` or `libfftw3f_threads` is
missing. This is because CMake considers a value false if it contains
`-NOTFOUND` at the end, but not if it's in the middle. For example,
CMake considers
`.../libfftw3f.a;.../libfftw3f_threads.a;FFTW3_LIBRARY_D-NOTFOUND` to be
false, but
`.../libfftw3f.a;FFTW3_LIBRARY_THREADS_F-NOTFOUND;.../libfftw3.a` to be
true.
---
I noticed that some other find modules also have the same list issue. I guess it was done this way to make CMake print all the found libraries instead of only the first.
Pull Request: https://projects.blender.org/blender/blender/pulls/136692
b1bb1d9815