> I believe the fix is to change line 3413 of configure.ac to read:
>
>     png_libs=`(libpng-config --ldflags) 2>&AS_MESSAGE_LOG_FD` || {

Thanks, I did something along those lines by installing the attached patch into 
master, and I'm boldly marking the bug report as done.

> although IMO that's not quite sufficient, because it doesn't set RPATH.

--ldflags is supposed to output any -R options needed, no? If it's not, it 
sounds like the libpng-config configuration is incorrect.