我有一个成功运行在iOS 7.1的iPhone上的应用程序。但是当我将部署目标更改为6.1并尝试在iPod touch上运行应用程序时，它会给我以下错误
Undefined symbols for architecture armv7: "std::__1::basic_streambuf<wchar_t, std::__1::char_traits<wchar_t> >::sync()", referenced from:
vtable for std::__1::basic_stringbuf<wchar_t, std::__1::char_traits<wchar_t>, std::__1::allocator<wchar_t> > in LibXL