No libldap_r in OpenLDAP 2.5 - use LDAP_API_FEATURE_X_OPENLDAP_REENTRANT from ldap_features.h

libpq does use libldap, coming from openldap. In the release
announcement for Openldap 2.5 ,
https://www.openldap.org/software/release/announce.html , is written
that libldap_r has been merged with libldap . In turn on a system
with OpenLDAP 2.4, after installing OpenLDAP 2.5, libldap is for v2.5
and libldap_r i for v2.4 . On such systems Libpq links with
libldap_r-2.4-releng.so.2 , while Postgresql Postgresql 13.4 is linked
with libldap-2.5 .

The release notes for Postgresql 13.4 ,
https://www.postgresql.org/docs/13/release-13-4.html, contain:

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<para>
Fix <application>configure</application> to work with OpenLDAP 2.5,
which no longer has a separate <filename>libldap_r</filename>
library (Adrian Ho, Tom Lane)
</para>

<para>
If there is no <filename>libldap_r</filename> library, we now
silently assume that <filename>libldap</filename> is thread-safe.
</para>
</listitem>

The current postgres’/configure.ac logic is: if libldap_r is availabe,
then use it, otherwise use libldap.

Searching with nm -D in libldap-2.4-releng.so,
libldap_r-2.4-releng.so, and libldap-2.5.releng.so I find that in the
former the symbol ldap_compare is not versioned, while the latter
contains ldap_compare@@OPENLDAP_2.5.releng . This could be used as
criterion, whether to skip ldap_r .

Moreover, the installed ldap_features.h file contains:

/* is -lldap reentrant or not */
#define LDAP_API_FEATURE_X_OPENLDAP_REENTRANT 1

/* is -lldap thread safe or not */
#define LDAP_API_FEATURE_X_OPENLDAP_THREAD_SAFE 1

I propose, instead of looking for the presence of libldap_r, to check
the feature macro, and if libldap is reentrant, not to use libldap_r .

Greetings
Дилян