http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51296
--- Comment #27 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-01-16 19:36:49 UTC --- C++11 requires that std::mutex::mutex() is constexpr, so that a global std::mutex will be statically initialized. If that constructor is constexpr it can't call pthread_mutex_init(). But C++11 also requires non-global, non-static mutexes to be possible, so we can't use the PTHREAD_MUTEX_INITIALIZER either. The option we've taken is to ignore the text I quoted in comment 25 and rely on the macro working for non-statically allocated mutexes, which works on most platforms.
