Big problem about libpq.so.2.1, ldconfig and ldd

How to solve this ? Please, please, it is urgent. We are in a big trouble!

We have noticed a strange behavior.
On a machine with 7.0.2, we have this :

ldconfig -p |grep libpq.so
        libpq.so.2.1 (libc6) => /usr/local/pgsql/lib/libpq.so.2.1
        libpq.so.2 (libc6) => /usr/local/pgsql/lib/libpq.so.2
        libpq.so (libc6) => /usr/local/pgsql/lib/libpq.so
 
On an other machine with 7.1.3, we have this :

ldconfig -p |grep libpq.so
        libpq.so.2 (libc6) => /usr/local/pgsql/lib/libpq.so.2
        libpq.so (libc6) => /usr/local/pgsql/lib/libpq.so

The interresting thing is the lack of the line : libpq.so.2.1 (libc6) => /usr/local/pgsql/lib/libpq.so.2.1

But, it is the source of our problems. Our programs compiled on the first machine do not work on the second one!
Well, not any more due to ???

Have you got an advice about this?

Thank you very much. (it is urgent)

Note : you can consult a previous mail of me about this.

--
Richard NAGY
Presenceweb / Nameshield