Only use AVX512 in own-FFTW if GROMACS also uses it

Only use AVX512 in own-FFTW if GROMACS also uses it

Building the own FFTW with AVX512 enabled for all AVX-flavors means that
an AVX2 build can end up loosing a significant amount of performance due
to clock throttle if the FFTW auto-tuner inadvertently picks and AVX512
kernel. This is not unlikely as measurements at startup are very noisy
and often lead to inconsistent kernel choice (observed in practice).

Change-Id: I857326a13a7c4dd1a6f5ab44360211301b05d3ac