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.h…
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.