如何强制xcode使用正确的iostream头文件

xcode 4.2
os x 10.7

当我把include设置成如下的样子时:

#include <iostream>
#include <boost/thread.hpp>

我从位于/opt/local/include/boost/tr1/tr1

Lexical or Preprocessor Issue
'boost/tr1/detail/config_all.hpp' file not found

/opt/local/include/boost/tr1/tr1/iostream文件内容:

//  (C) Copyright John Maddock 2005.
//  Use, modification and distribution are subject to the
//  Boost Software License, Version 1.0. (See accompanying file
//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
//  This file exists to prevent std lib headers from accidentally
//  including a TR1 extention header; we must suppress this otherwise
//  we can end up with cyclic dependencies with some std lib implementations.
//
#ifndef BOOST_TR1_iostream_INCLUDED
#  define BOOST_TR1_iostream_INCLUDED
#  ifndef BOOST_TR1_NO_RECURSION
#     define BOOST_TR1_NO_RECURSION
#     define BOOST_TR1_NO_iostream_RECURSION
#  endif
#  include <boost/tr1/detail/config_all.hpp>
#  if defined(BOOST_HAS_INCLUDE_NEXT) && !defined(BOOST_TR1_DISABLE_INCLUDE_NEXT)
#     include_next <iostream>
#  else
#     include BOOST_TR1_STD_HEADER(iostream)
#  endif
#  ifdef BOOST_TR1_NO_iostream_RECURSION
#     undef BOOST_TR1_NO_iostream_RECURSION
#     undef BOOST_TR1_NO_RECURSION
#  endif
#endif

我应该使用位于/Developer/SDKs/MacOSX10.7.sdk/usr/include/c++/4.2.1/的iostream标头

你必须原谅我，因为我对c++很陌生，对xcode更陌生。提前感谢您的任何帮助。