pgsql: 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

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/29a4fbd468af3b627852b6d72d712c47baaef1b6

Modified Files
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configure    | 27 ++++++++++++++-------------
configure.in |  5 +++--
2 files changed, 17 insertions(+), 15 deletions(-)