Diagnose incompatible OpenLDAP versions during build and test.

With OpenLDAP versions 2.4.24 through 2.4.31, inclusive, PostgreSQL
backends can crash at exit.  Raise a warning during "configure" based on
the compile-time OpenLDAP version number, and test the crash scenario in
the dblink test suite.  Back-patch to 9.0 (all supported versions).

contrib/dblink/input/paths.source

@@ -0,0 +1,14 @@
+-- Initialization that requires path substitution.
+
+CREATE FUNCTION putenv(text)
+   RETURNS void
+   AS '@libdir@/regress@DLSUFFIX@', 'regress_putenv'
+   LANGUAGE C STRICT;
+
+CREATE FUNCTION wait_pid(int)
+   RETURNS void
+   AS '@libdir@/regress@DLSUFFIX@'
+   LANGUAGE C STRICT;
+
+CREATE FUNCTION set_pgservicefile(text) RETURNS void LANGUAGE SQL
+    AS $$SELECT putenv('PGSERVICEFILE=@abs_srcdir@/' || $1)$$;