Define LDAP_NO_ATTRS if necessary.

Commit 83aaac41 introduced the use of
LDAP_NO_ATTRS to avoid requesting a dummy attribute when doing search+bind
LDAP authentication.  It turns out that not all LDAP implementations define
that macro, but its value is fixed by the protocol so we can define it
ourselves if it's missing.

Author: Thomas Munro
Reported-By: Ashutosh Sharma
Discussion: https://postgr.es/m/CAE9k0Pm6FKCfPCiAr26-L_SMGOA7dT_k0%2B3pEbB8%2B-oT39xRpw%40mail.gmail.com

src/backend/libpq/auth.c

@@ -2398,6 +2398,11 @@ InitializeLDAPConnection(Port *port, LDAP **ldap)
 #define LPH_USERNAME "$username"
 #define LPH_USERNAME_LEN (sizeof(LPH_USERNAME) - 1)
 
+/* Not all LDAP implementations define this. */
+#ifndef LDAP_NO_ATTRS
+#define LDAP_NO_ATTRS "1.1"
+#endif
+
 /*
  * Return a newly allocated C string copied from "pattern" with all
  * occurrences of the placeholder "$username" replaced with "user_name".