pgsql: Update configure's probe for libldap to work with OpenLDAP 2.5.

Update configure's probe for libldap to work with OpenLDAP 2.5.

The separate libldap_r is gone and libldap itself is now always
thread-safe. Unfortunately there seems no easy way to tell by
inspection whether libldap is thread-safe, so we have to take
it on faith that libldap is thread-safe if there's no libldap_r.
That should be okay, as it appears that libldap_r was a standard
part of the installation going back at least 20 years.

Report and patch by Adrian Ho. Back-patch to all supported
branches, since people might try to build any of them with
a newer OpenLDAP.

Discussion: https://postgr.es/m/[email protected]

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7e0b53ad6d99ad30c54b0586e28e39e7667712de

Modified Files
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configure | 89 +++++++++++++++++++++++++++-------------------
configure.in | 20 ++++++-----
src/include/pg_config.h.in | 3 --
3 files changed, 64 insertions(+), 48 deletions(-)