Add LDAP_API_FEATURE_X_OPENLDAP to imply this implementation

has OpenLDAP extensions.  (This may be defined on non-OpenLDAP
implmentations that implement features of our API).
The extensions implied by this flag are TBD.

include/ldap.h

@@ -44,6 +44,9 @@ LDAP_BEGIN_DECL
 /* We'll eventually release as 200 */
 #define LDAP_VENDOR_VERSION	190
 
+/* OpenLDAP API Features */
+#define LDAP_API_FEATURE_X_OPENLDAP 1
+
 /* include LDAP_API_FEATURE defines */
 #include <ldap_features.h>