c++11 question: clang++ 3.3 <future> header not found

[Quark]

% uname -a
FreeBSD cobalt 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

test program
#include <iostream>
#include <future>

int main( int argc, char* argv[])
{
        auto f = std::async( [] () {
                        std::cout << "Hello, World!" << std::endl;
                        });
        f.wait();
        return 0;
}


error received is
% clang++ -otest test.cc

test.cc:2:10: fatal error: 'future' file not found
#include <future>
         ^
1 error generated.

I guess clang is re-using system headers which belong to older gcc 4.2
I also have gcc48 installed, how can I make clang to refer gcc48 headers?