cmake: fix header install customization (#58) #148

Job	Run time
heffte (cmake, BUILTIN, cpu_intel)	1m 12s
heffte (cmake, FFTW, cpu_intel)	2m 6s
heffte (cmake, MKL, cpu_intel)	1m 44s
heffte (spack, BUILTIN, cpu_intel)	12m 4s
heffte (spack, FFTW, cpu_intel)	16m 38s
heffte (spack, MKL, cpu_intel)	16m 29s
heffte (cmake, ONEAPI, gpu_intel)	3m 45s
heffte (cmake, CUDA, gpu_nvidia)	7m 51s
heffte (spack, CUDA, gpu_nvidia)	19m 36s
heffte (cmake, ROCM, gpu_amd)	41s
	1h 22m 6s

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