createlang plpgsql error

I'm having trouble installing plpgsql. Here's the error:

[postgres(at)shimzilla home]$ createlang plpgsql template1
ERROR: stat failed on file '$libdir/plpgsql': No such file or directory
createlang: language installation failed

[postgres(at)shimzilla home]$psql template1
Welcome to psql 7.3, the PostgreSQL interactive terminal.
template1=# select version();
version
-----------------------------------------------------------
PostgreSQL 7.3 on i686-pc-linux-gnu, compiled by GCC 2.96
(1 row)

Some more info:
This is on a RedHat 7.3, with Postgresql 7.3, on an intel box.

I built from source, and configured with one option:
./configure --prefix=/opt/pgsql

After I configured I had to edit /etc/ld.so.conf adding the line:
/opt/pgsql/lib

Then I ran ldconfig.

I installed in /opt instead of /usr/local because I want this install to
be the same as a solaris install. In fact I first installed in /usr/local
with no problems at all before deciding that overall it'd be better to
have the installs on the two servers be as identical as possible. The
first install was removed with "make uninstall"

Any ideas or pointers to urls?

Thanks in advance,
Jim Martinez