I traced the build failure to a Debian patch from Debian bug report 357775. The specific patch is 11_fix_get_libdir_name.diff, which can be viewed here, about midpoint in the page:
http://lists.alioth.debian.org/pipermail/pkg-kde-commits/2006-June/003702.ht...
I reversed the patch and then was able to build on 64-bit. Ignoring obvious file size differences, my 64-bit package looks exactly the same as my 32-bit package.
Push it. That patch should never have found its way into GIT in the first place.
Pushed in GIT hash 32d597f9.
Darrell