pgsql: Un-break AIX build, take 2.

Un-break AIX build, take 2.

I incorrectly diagnosed the reason why hoverfly is unhappy.
Looking closer, it appears that it fails to link libldap
unless libssl is also present; so the problem was my
idea of clearing LIBS before making the check.  Revert
to essentially the original coding, except that instead
of failing when libldap_r isn't there, use libldap.

Per buildfarm member hoverfly.

Discussion: https://postgr.es/m/17083-a19190d9591946a7@postgresql.org

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/56533fce7345669b7f0b5f9f4467ab81b19fbe19

Modified Files
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configure    | 97 ++++++++++++++++++++++++------------------------------------
configure.in | 26 ++++++++--------
2 files changed, 51 insertions(+), 72 deletions(-)