Re: Fix "warning: cast from pointer to integer of different size"

[Juan Lang]

> 3) Add a definition if FIELD_OFFSET wrapped in #ifndef. This is the
>     least ugly option but would cause surprising results if one or
>     the other was changed.

Personally, I think this looks the best.  As far as which is likely to
get accepted.. often you have to try first to find that out ;-)
--Juan