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 :)