call_once: deprecate (Function, once_flag&) overload

[raad@…]

The compatibility overload of call_once with reversed argument order leads to confusing error messages in clang.

The reason is that clang tries to instantiate template<typename Function> inline void call_once(Function func,once_flag& flag) with Function = boost::once_flag.

Could that overload be removed for the next BOOST_THREAD_VERSION? That would also help with codebases which need to use boost::call_once and std::call_once interchangeably as this overload is not present in the std version.