Fix AX_OPENMP performance regression

When moving from AC_OPENMP and AX_OPENMP we stopped AC_SUBST of OPENMP_CFLAGS and these haven't been used since. Fix that, but do that by only using the OPENMP_CFLAGS as needed, that is as compiler hints for vectorizing some loops in the gravity interactions and only requiring the OpenMP runtime when linking against an OpenMP FFTW library.

Fixes #865 (closed)