BUILD_OWN_FFTW triggers no-SSE warning
Due to my change d33720e6 setting BUILD_OWN_FFTW generates a warning about no SSE being enabled in FFTW. This is because I replaced the positive test on AVX by a negative test on SSE and AVX_128. Apparently BUILD_OWN_FFTW sets HAVE_SIMD somewhere. I don't fully understand how this works, so it might be better if someone else fixes this.
Fix own-FFTW build
Recent changes broke the expectation that the own-FFTW build attempts
to pretend to have run SIMD-support tests on the build that it has not
yet performed. This is all more complicated than it needs to be,
and will be refactored in master branch.
Refactor FFTW setup
It proved bug-prone to have both own-FFTW and find-FFTW mechanisms try
to fulfil the same contract, and it was not needed in the first
place. We do not need to fake the results of SIMD tests just to avoid
our own warnings (and that was all that they were used for; mdrun does
further SIMD-support detection at run time after possible dynamic
linking of FFTW.
Now we check when there is something to check, and not when there is
not. Removed TODOs and comments no longer appropriate for the