favor lookup for threaded fftw libraries #152

Job	Run time
heffte (cmake, BUILTIN, cpu_intel)	1m 21s
heffte (cmake, FFTW, cpu_intel)	1m 55s
heffte (cmake, MKL, cpu_intel)	1m 50s
heffte (spack, BUILTIN, cpu_intel)	16m 11s
heffte (spack, FFTW, cpu_intel)	16m 25s
heffte (spack, MKL, cpu_intel)	16m 17s
heffte (cmake, ONEAPI, gpu_intel)	1m 34s
heffte (cmake, CUDA, gpu_nvidia)	8m 43s
heffte (spack, CUDA, gpu_nvidia)	20m 3s
heffte (cmake, ROCM, gpu_amd)	40s
	1h 24m 59s

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