Fix make test_pam_modules

Fix make test_pam_modules

In Samba3 there is a very handy Makefile target 'test_pam_modules'. It let
quickly veryfy, that obtained PAM module actually is loadable and doesn't
miss any dependency libs. the only problem that on FreeBSD it doesn't work
OOTB, as it unconditionally adds -ldl to the list of libraries when FreeBSD
doesn't have it and doesn't need it.

This small patch fixes the problem for FreeBSD and, I hope, still valid
for othe systems, where -ldl is required. Has to be tested there though.

With regards,
Timur Bakeyev.

Signed-off-by: Stefan Metzmacher <metze@samba.org>