using clang with gcc 4.8 headers

I am trying to use clang (release 3.1) under Mac OS X 10.8.2
with gcc 4.8 toolchain.

I have compiled llvm/clang from scratch with --with-gcc-toolchain=<my gcc 4.8 path>

However, when I try to compile my source code, clang doesn’t get headers
from the specified path <my gcc 4.8 path/include. Instead, it looks in default
directory and pulls in default gcc 4.2 headers.

What am I doing wrong and how to fix this?