- VectorMath_SSEx.c requires _mm_add_pd() which is an SSE2 intrinsic
  (defined in emmintrin.h) and therefore cannot be build on platforms
  with support for only SSE1 (e.g. at least some i386 systems)
- Raise explicit error in VectorMath_SSEx.c if __SSE2__ is not defined
- Remove SSE1 from supported instruction sets in VectorMath_internal.c
  and VectorMath.c
- Remove SSE1-specific build of VectorMath module in Makefile.am