BUG #17083: [PATCH] PostgreSQL fails to build with OpenLDAP 2.5.x

The following bug has been logged on the website:

Bug reference: 17083
Logged by: Adrian Ho
Email address: ml+postgresql(at)03s(dot)net
PostgreSQL version: 13.3
Operating system: Ubuntu 20.04.2 LTS
Description:

OpenLDAP 2.5 merged the `ldap_r` and `ldap` libraries, so all per-handle
routines are now thread-safe by default. However, the removal of `ldap_r`
results in the following `configure` error:
```
checking for ldap_bind in -lldap... yes
checking for ldap_simple_bind in -lldap_r... no
configure: error: library 'ldap_r' is required for LDAP
```
This patch therefore fixes and clarifies the LDAP library detection
logic, by first selecting on thread-safety requirements, then looking
for the appropriate libraries. The underlying assumption is that for a
pre-2.5 setup, both `ldap` and `ldap_r` libraries are installed, so it
should only fall back to (thread-safe) `ldap` in a 2.5.x installation.

diff --git a/configure.in b/configure.in
index 14a6be6..02ad260 100644
--- a/configure.in
+++ b/configure.in
@@ -1241,17 +1241,19 @@ fi
if test "$with_ldap" = yes ; then
_LIBS="$LIBS"
if test "$PORTNAME" != "win32"; then
- AC_CHECK_LIB(ldap, ldap_bind, [],
- [AC_MSG_ERROR([library 'ldap' is required for LDAP])],
- [$EXTRA_LDAP_LIBS])
- LDAP_LIBS_BE="-lldap $EXTRA_LDAP_LIBS"
if test "$enable_thread_safety" = yes; then
# on some platforms ldap_r fails to link without PTHREAD_LIBS
- AC_CHECK_LIB(ldap_r, ldap_simple_bind, [],
- [AC_MSG_ERROR([library 'ldap_r' is required for LDAP])],
+ # OpenLDAP 2.5 merged ldap_r with ldap
+ AC_SEARCH_LIBS(ldap_simple_bind, [ldap_r ldap], [],
+ [AC_MSG_ERROR([not found in any LDAP library])],
[$PTHREAD_CFLAGS $PTHREAD_LIBS $EXTRA_LDAP_LIBS])
- LDAP_LIBS_FE="-lldap_r $EXTRA_LDAP_LIBS"
+ LDAP_LIBS_BE="-lldap $EXTRA_LDAP_LIBS"
+ LDAP_LIBS_FE="${ac_lib:+-l}$ac_lib $EXTRA_LDAP_LIBS"
else
+ AC_CHECK_LIB(ldap, ldap_bind, [],
+ [AC_MSG_ERROR([library 'ldap' is required for LDAP])],
+ [$EXTRA_LDAP_LIBS])
+ LDAP_LIBS_BE="-lldap $EXTRA_LDAP_LIBS"
LDAP_LIBS_FE="-lldap $EXTRA_LDAP_LIBS"
fi
AC_CHECK_FUNCS([ldap_initialize])