FreeBSD 9.0-CUR/amd64 CLANG: howto use gcc __builtin_ia32?

[Hartmann, O.]

I use some numerical code utilizing the SIMD units of modern X86 
architectures. Code compiles well using gcc/gcc46,
but clang does not know about the __builtin_ia32_xxxxx() statements. How 
to treat those in clang and how to make
C code compiling with clang utilizing those __builtin_ia32 statements?

Thanks,
Oliver