Use the autotools macro for vectorization

The configure script defines the macros HAVE_SSE, HAVE_AVX, HAVE_AVX2 and so on. We would like to use these in vector.h instead of __AVX__. That would make everything self-consistent and prevent us from having to edit the code to switch vectorisation on or off.

Note that for this to work we need to make sure the configuration script disables the definition of these macros when configuring with --disable-vec.