Clang 3.2(Apple Clang 4.2) error using std::atomic

On compiling a project with a pre-compiled header including <atomic> on Mac OSX 10.8.2 , I am getting this c++11 related error ,

#include ../xxx/pch.h:71:10: fatal error: 'atomic' file not found
#include <atomic>
^
1 error generated.
make[1]: *** [debug/xxx/objective-c++.pch] Error 1
make: *** [debug] Error 2
11:32:01: The process "/usr/bin/make" exited with code 2.
Error while building/deploying project xxx (kit: Qt5.1.0)
When executing step 'Make'

I have included the below flags in the.pro file to enable c++11 ,

QMAKE_CXXFLAGS += -std=c++11

CONFIG += c++11

I am building the project with the pre-built QT5.1Beta package (qt-mac-opensource-src-5.1.0-beta1.tar.gz) which I believe has c++11 enabled.

I use QT Creator to build the project. Compiler used is Clang(x86 64bit in /usr/bin).
I have the latest version of Xcode 4.6.2 and the command line tools installed.

$ clang++ -v 
Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn) 
Target: x86_64-apple-darwin12.2.1
Thread model: posix

I see the atomic header in the below locations /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/c+/v1/atomic and in /usr/lib/c+/v1/atomic

I tried testing by including the complete path , /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr//lib/c++/v1/atomic

I am getting the below error,

In file included from ../xxx/pch.h:71:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/c++/v1/atomic:535:2: 
error: atomic is not implemented

In file included from ../xxx/pch.h:124:
../../../yyy.h:308:12: error: no type named 'atomic' in namespace 'std'
std::atomic<bool> m_signaled;

Attached is the screenshot of my QtKit configuration