Windows SSE2 flag added

There are many options for Windows SSE2 flags, most of which are not
optimal. We'd rather have a false positive (possibly resulting in a
compiler error that we could then expore further) than a false negative.

_M_IX86_FP == 2 will tell you that the compiler flags are set to SSE2,
but they are often not set and SSE2 would still build properly if we
allow it.

_M_IX86 || _M_X64 would assume that all Intel CPUs had SSE2, but would
incorrectly include the headers on old versions of Visual Studio.

_MSC_VER >= 1300 seems to be the best test. This corresponds to Visual
Studio .NET 2002, the first version that had SSE2.

_M_CEE_PURE also must be off. Google it.