[boost] [system] Do we need this library with latest (almost C++0x) compilers?

Can somebody please explain what is missing in current compiler implementations? What is preventing other libraries to have a macro like: #ifdef _CPP11_THREAD_     #include <thread>     #include <mutex>     #include #else     #include     #include     #include #endif  also when it comes to error codes and conditions? Thanks, Tony