c++11 compliance: Adapt the one_flag and call_once to the c++11 interface

[viboes]

The current implementation is doesn't complies completely with c++11 standard 30.4.4 Call once.

struct once_flag { 
  constexpr once_flag() noexcept;
  once_flag(const once_flag&) = delete; 
  once_flag& operator=(const once_flag&) = delete;
};

E.g.

boost::once_flag once;

fails

and needs

boost::once_flag once=BOOST_ONCE_INIT;

In addition one_flag in not copyable.

[viboes]

committed diffs in trunk [77767]