[Qt-creator] Make QtCreator know that __SSE2__ is defined

[Daniel Teske]

> However, as QTCreator does not know that __SSE2__ is declared at compile
>  time ( -march pentium4 ), all is grayed out in the #idef block, and I have
>  no quick access to functions, no error highlighting, etc...

If I'm correctly assuming your compiler defines __SSE2__ depending on the 
passed on march parameter?

Then that's currently not possible. 

Though depending on how you set "-march pentium4", we might be able to pass 
that on to our toolchain classes.

daniel