Fix busted test for ldap_initialize.
Sigh ... I was expecting AC_CHECK_LIB to do something it didn't,
namely update LIBS. This led to not finding ldap_initialize.
Fix by moving the probe for ldap_initialize. In some sense this
is more correct anyway, since (at least for now) we care about
whether ldap_initialize exists in libldap not libldap_r.
Per buildfarm member elver and local testing.
Discussion: https://postgr.es/m/17083-
a19190d9591946a7@postgresql.org