cmake: Fix linking of pfft to FFTW-MPI

Octopus does not use FFTW-MPI, but pfft does, and does not link to it.

@LecrisUT opened an issue on the upstream Github page, but it looks abandoned. As such, the options are (to be explored in this order):

• Octopus searches for FFTW-MPI and makes that available for linking if pfft is required. I'm not specifically sure what this means, maybe some modification of our pfft target in cmake might work

• Patch pfft's build system and maintain a fork

  • Note, this means we have to not use find_package for pfft

• Require pfft and pnfft together, as it sounds like linking works out in this case