[boost] Best home for cxx11_char_types.hpp?

I'm working on an update of Boost.Filesystem that adds support for the C++11 char16_t, char23_t, u16string, and u32string types. For compilers that don't yet support these types, a header cxx11_char_types.hpp (see attached) provides emulation that is good enough for filesystem, and is probably useful for other libraries and for user code too. We discussed this header several years ago, and I've been using it since then. Where should this header live? It can certainly go in libs/filesystem/include/boost/filesystem, but that doesn't really expose it to general Boost users. Is there a better place, but without introducing unwanted dependencies? --Beman