P.S.  Chris, you wrote:  It's probably that it couldn't find libpq to
link against, or some
such thing.

I did run the configure under strace and I see it opens libpq.so.

Just to double check, I tried setting gcc's LIBRARY_PATH variable:

LIBRARY_PATH=/opt/PostgreSQL/8.4/lib  ./configure
--with-pgconfigdir=/opt/PostgreSQL/8.4/bin/
--prefix=/opt/slony1-1.2.21
...
checking for PQunescapeBytea in -lpq... no
configure: error: Your version of libpq doesn't have PQunescapeBytea
     this means that your version of PostgreSQL is lower than 7.3
     and thus not supported by Slony-I.

Best,
-at