undefined reference to crypt

Hi,

I am discovering PostGreSQL and Linux.
I have a little problem with libpq.

Here is my C program (it's empty!)

/* --------- essai.c -------------------------- */

#include <stdio.h>
#include "/usr/lib/pgsql/include/libpq-fe.h"

int main(void)
{

}

/* ------------------------------------------- */

Here is my makefile:

essai.o: essai.c
gcc -c essai.c
essai: essai.o
gcc -o essai essai.o -lpq

When I do "make essai", the following error appears:

/usr/lib/libpq.so: undefined reference to crypt.
collect2: ld retruned 1 exit status

I suppose a file/library is missing.
Can you help me?

The postgres' programmer guide refers to several examples in
/src/test/regress.
These samples are not installed with my postgres distribution .
Where can I download them (or other examples?).

Sorry for my poor english and thank you for your answers,

Patrice DUMAS
Ardèche, FRANCE
pdumas(at)inforoutes-ardeche(dot)fr