Problems compiling with libpq

I know I am being silly, but it is late and I cannot recall what the
solution was...

When I write a C program with
#include <libpq-fe.h>
or
#include "libpq-fe.h"

and compile with -llibpq turned on, I get the message that the system
cannot find libpq-fe.h
I know it is me being really silly, for the headerfile is where I expected
it: /usr/local/pgsql/include/libpg-fe.h
The solution should be to tell gcc where to find this file... even better
would be: to include the dir to the include-files path... but there it
stops for me, anyone who can help me out?

Gcc 2.95.3, Slackware 8.1, PostgeSQL 7.3 ordinary users as well as root
should be able to compile it. (some little voice in my head told me that it
might differ for root)

tia,
Michiel