Update for FFTW 3.3.5

Update for FFTW 3.3.5

The auto-download now gets FFTW 3.3.5 and builds it properly,
including with --enable-vsx when GMX_SIMD is set to VSX, i.e. for
Power8, and --enable-avx512 when GMX_SIMD is any of the AVX flavours
(which is safe on non-512 now, works on KNL, and is presumed useful
for future AVX512 architectures).

Note that 3.3.5 does not automatically add avx and sse support if
configured with --enable-avx2, so changed the documentation
accordingly.

Change-Id: I2a48ec1cb4f812dcae18870e0afcaf1120da251b