createlang doesn't work

Hello.
What do I have to do to install plpgsql ?

I tried as root:
/usr/local/pgsql/bin/createlang plpgsql test
to enable plpgsql in database test, but I always get the message:
language installation failed: ERROR: could not access file
"$libdir/plpgsql": File or dir not found.

$libdir is undefined and the file plpgsql is not on my HD. Do I
eventually have to change ./configure-options somehow? I cannot find
./configure options for this, so which one can I add?

Thanks for help,
Robert