[boost] [config] libs not supporting std::u16string and std::u32string.

Hi, compiling Boost.Ratio/io Boost.Chrono/io with compilers supporting |char16_t and |char32_t fails if the library don't provides std::u16string and std::u32string. Could we add a macro |BOOST_NO_CXX11_HDR_STRING||| (c++11 is not completely provided) or the two more specific |||BOOST_NO_CXX11_U16STRING||| and |||BOOST_NO_CXX11_U32STRING||| for these cases? Best, Vicente || ||