Manually check for SSE3 support at startup

I just started having a look at the code. The first "FIXME" tag I found was one requesting "manual" check of SSE3. Hopefully my future patches will be more useful :)

I tested this on a new (Core Duo) and old (Pentium III) CPU. Lacking of SSE3 support is correctly detected in the second case.

This is my first patch _ever_ to an open source project. Be nice :)
Ok, last thing and i'm merging: drop this brace pair (the outermost in
```
#else
{ <- this
  ...
} <- this
#endif
    if (!sse3_supported))
```
 and reindent "...".