[boost] patch against boost threads library

Short notes: * thread_exception::message() is broken beyond repair, as it returns a pointer to local storage (via std::string::c_str()) * any program using it is broken, too. * it is unused by boost itself(!) which is why it can be removed * it is the only reason why a helper-function 'system_message()' is needed, which can then be removed, too. * system_message() is the only reason several headers and a hack for strerror() were needed, which can then also be removed. * system_message() requires a char-based API on win32, which is not available under MS Windows CE, strerror() is also not available there, making two more reasons to remove it. cheers Uli