Separate the hand-written vectorization and the auto-vectorization flags

On platforms where we do not have hand-written intrinsics (yet) we must configure the code by disabling the vectorization. At the moment this also implies remove all the compiler flags that deal with vectorization.

Could we add a flag --disable-hand-vec or something similar that just undefined WITH_VECTORIZATION in the config.h ? --disable-vec would then keep its current behaviour of disabling this define and removing the compiler flags.