cleanup for imkl v11.1.x FFTW wrapper support

@hajgato: cleanup for your PR https://github.com/hpcugent/easybuild-easyblocks/pull/445

Seems to be working, but needs some more testing (i.e. all existing `imkl` supported installs needs to be retested).

The diff here is going to be pretty confusing, the diff in your PR with this merged in should be clearer.