bson.h does not compile when included on its own

I'm filing this separately from SERVER-1267, since that is a general request to make all the client facing headers compile cleanly when included in isolation. But currently bson.h is unusable from client code, in a way that does not seem be solvable by including other mongo headers before it, and it is critical that bson.h work since it is the main header for C++ clients using BSON.