Un-break AIX build.

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