Un-break AIX build.
In commit
d0a02bdb8, I'd supposed that uniformly probing for
ldap_bind would make the intent clearer. However, that seems
not to work on AIX, for obscure reasons (maybe it's a macro
there?). Revert to the former behavior of probing
ldap_simple_bind for thread-safe cases and ldap_bind otherwise.
Per buildfarm member hoverfly.
Discussion: https://postgr.es/m/17083-
a19190d9591946a7@postgresql.org