Avoid ifdef by using %td printf format

Dear Peter,

Don't think I have abandoned you. :-) I like knowing about bellbird progress so I subscribed to it, and I try to follow your contributions as closer as I can.

I saw you introduced a "#ifdef" for printing a pointer substraction result and I found this "cleaner" solution:

Use %td format for ptrdiff_t data types (see: http://stackoverflow.com/questions/3597743/where-is-ptrdiff-t-defined-in-c )

It's just a tiny commit, but I hope you like it.

Best regards!


PS: By the way, it is amazing the number of format modifiers printf has! http://en.wikipedia.org/wiki/Printf_format_string