cmake: allow to customize install locations (#57) #146

Job	Run time
heffte (cmake, BUILTIN, cpu_intel)	1m 11s
heffte (cmake, FFTW, cpu_intel)	1m 51s
heffte (cmake, MKL, cpu_intel)	1m 41s
heffte (spack, BUILTIN, cpu_intel)	11m 57s
heffte (spack, FFTW, cpu_intel)	22m 18s
heffte (spack, MKL, cpu_intel)	12m 3s
heffte (cmake, ONEAPI, gpu_intel)	1m 33s
heffte (cmake, CUDA, gpu_nvidia)	6m 24s
heffte (spack, CUDA, gpu_nvidia)	16m 0s
heffte (cmake, ROCM, gpu_amd)	45s
	1h 15m 43s

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