This line will not detect when mkl is provided by intel-oneapi-mkl

For example:

spack install [email protected]+cuda~mpi~shared cuda_arch=61,70,75,80,86 ^[email protected] ^intel-oneapi-mkl

will set -DGMX_FFT_LIBRARY=fftw3 instead of -DGMX_FFT_LIBRARY=mkl

@danielahlin @eirrgang @junghans @haampie