[boost] format lib patch

Config: STLport 5.0 beta and MSVC 8 (but also reported with 7.1) It looks like this compiler has a problem with the relation between the basic_string::size_type and basic_format::size_type as soon as the library is not the native one. Here is a patch proposition that return a basic_string::size_type rather than the basic_format::size_type. I think it is quite a secure patch as the method is already using a static_cast<typename string_type::size_type> to return its result and not a simple static_cast<size_type>. Bests