clang and c++11 question

[Quark]

% uname -a
FreeBSD cobalt.corp.nai.org 9.2-RC3 FreeBSD 9.2-RC3 #0 r254795: Sat Aug 24 20:25:04 UTC 2013     root at bake.isc.freebsd.org:/usr/obj/usr/src/sys/GENERIC  amd64


% clang++ --version
FreeBSD clang version 3.3 (tags/RELEASE_33/final 183502) 20130610
Target: x86_64-unknown-freebsd9.2
Thread model: posix

clang 3.3 is C++11 feature complete, but it fails to find <future> and/or <thread> headers.
since it is looking in system compiler path, which is old gcc4.2, not C++11 ready

how to make clang refer headers from gcc48's installation( via pkg_add)?