C++11 feature not valid when build ios package through unrealbuildtool

in our project, i want use std::atomic feature which is define in file, it is ok when i build android package, but ios package is failed and report:
error: ‘atomic’ file not found
my environment:
Apple LLVM version 6.0 (clang-600.0.51) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin13.4.0
ant i found the file is at this path “/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/atomic”,
but i do not know how to add this path to the include path which unrealbuildtool can find, please help me!

How is it “ok when i build android package” ??? The issue " i want use std::atomic" is exactly my case, and i am unable to package Android! what did you do?