Disallow the use of Clang to compile AVX_128_FMA

Clang apparently produces buggy code both for verlet kernels
and group kernels when the AVX_128_FMA acceleration is enabled.
Since it happens in two places it is unlikely to be a minor
bug, and since it covers versions up to the currently
released one, we don't have much choice but to disallow this
combination for now.
Fixes #1099.

Change-Id: If8346fa0a58f51a0ef6ac16e4801fe98ad1c73c9