[boost] [filesystem] version 3 use of std::mbstate_t

I believe that the version 3 filesystem headers which use std::mbstate_t need to include cwchar, but that they do not. This gives an error when I compile using Comeau with a VC9 backend, although compiling directly with VC9 is OK. This probably happens because of the assumption in filesystem that cwchar is included by some other C++ header, but rather than make such an assumption it would be better if cwchar were included directly by any header file which uses std::mbstate_t.