Thanks for the comments. It's indeed the multiple libjpeg issue I'm taking a stab at here. Your fix predates 10.3 while my suggested tweak requires a 10.3+ ld feature (-undefined dynamic_lookup) so perhaps what seemed a necessity back then is better handled in this fashion nowadays. And who knows when the 10.0 compatibility mode disappears?
I can try this fix on a few more Macs before passing final judgement. I've got ppc32 on 10.4, x86 on 10.4 and x86_64 on 10.5 in Roxen's build farm.
Unfortunately I couldn't get the #pragma idea to work so it's not an option.