Could not find your PostgreSQL installation in /usr/lib/postgresql/8.1

Hi all,
I'm using kubuntu where I've installed the postgresql using apt-get (8.1), and
now I'm trying to compile pgadmin but I got this error while
running ./configure:
Could not find your PostgreSQL installation in /usr/lib/postgresql/8.1

I guess my postgresql directory should be /usr/lib/postgresql/8.1, that
contains:
luca(at)fluca:/usr/lib/postgresql/8.1]$ ls -l
totale 2
drwxr-xr-x 2 root root 816 2006-11-17 18:33 bin
drwxr-xr-x 2 root root 1936 2006-11-17 18:33 lib

I tried running ./configure with the --with-pgsql, specifying the above
directory but nothing changed. I took a look at the configure script and
found that the above message is echoed when pg_config is not found, and I
cannot find it on my system. Do I have to install a specific package to get
this program? How can I compile pgadmin?

Thanks,
Luca