Fix FindFFTW behaviour

Fix FindFFTW behaviour

FFTW 3.3.5 with --enable-avx* will enable any useful 128-bit
SIMD flavours by default. (See Erik's 579cec9a6 in their repo.) Our
detection code will observe this, and will be silent.

With earlier FFTW, if we're doing a GROMACS AVX build, and we have
FFTW SIMD, and not SSE(2) SIMD, then we want to warn the user to
reconfigure FFTW to add SSE support. Made this behaviour correct, and
minimized the necessary infrastructure for it.

Added detection support for some other SIMD-support symbols that
are present in the FFTW repo for upcoming hardware.

Fixes #1809

Change-Id: If586250895664581316505a5595da7442e789f8d