Thanks to Dominus, I've got my build procedure working perfectly again. In fact, I now cannot make it fail on my standard machine or on either of the two OS X installations on my borrowed machine.
I've gone through my script and put back (one by one) the EXPORT commands that Dominus doesn't use; that didn't cause any problems.
The original problem had to do with png - and Dominus's suggestions about modifying the Makefile fixed that problem.
I'm very sorry to have taken up so much time and bandwidth on a problem that seems to have affected only my setup. But I'm very grateful for help in getting my project going again, because I want to include in it the last four months of fixes in svn.
Thank you again! As before, the results of this project will be available here:
http://www.columbia.edu/~em36/wpdos/wpdosmac-dosbox.html
and the Windows-based counterpart is here:
http://www.columbia.edu/~em36/wpdos/wpdosbox64.html