Hmm. Can't figure out why you're running into that compile problem. Based on configure for libdbi-drivers here (also Ubuntu 8.10), the PostgreSQL include file test checks for libpq-fe.h in /usr/include/postgresql - and libpq-fe.h is supposed to be installed as part of the libpq-dev package.
Maybe not much help, but files similar to this are located at /usr/include/postgresql/8.3/server/libpq
Would it help to use apt to install anything first?
