Where is PostgreSQL root directory?

Hello:

This is the simplest question on this form but I have pulled all my hair
trying figuring it out. PLEASE HELP!

I have installed PostgreSQL V8.3.7 (RPM) on a CentOS 5.2 system.
PostgreSQL server STARTS & STOPS fine.

Now I am trying to install "pgAdmin" (having already installed
wxWigets). When I issue the "./configure" command to install "pgAdmin",
I get an error message saying that it can't find the PostgreSQL and that
I should use the --with-pgsql=DIR command.  Fine I issue the following
command:

%./configure  --with-pgsql=/usr/lib     ;(/usr/lib is where I see the
root directory)

but I still continue to get the same error. The documentation for
PostgreSQL mentions that the root directory is "/usr/local/pgsql". I
tried that. Again the same error.

What am I doing wrong? Using RPM PostgreSQL installation, where is the
root directory for pgsql?

This all seems simple but I am struck. PLEASE HELP!

Sad SAM