Good news: I have 2 solutions, so I'm not asking anybody to do any work! I hope this might somebody else - or another reader may wish to share another solution.
////// The Issue //////
The MakeMaker tool (perl Makefile.PL; make ; sudo make install) wants to build a "universal" binary (compiled for i386 and x86_64). However, libjpeg-7 wouldn't build as a universal - so we get linker errors. I think (although I'm not certain), that MakeMaker "learns" its archflags from the file /System/Library/Perl/5.12/darwin-thread-multi-2level/Config_heavy.pl
////// My Environment //////
I keep the Imager-0.87 and jpeg-7 in the ~/gnu directory (however the name of the directory is of no significance)