Mark the `MYSQL_NEW_LINK` constant as deprecated in 3.9+ as no equivalent to the `$new_link` parameter exists in `mysqli_*` functions.

Fixes #27933.

Built from https://develop.svn.wordpress.org/trunk@28657


git-svn-id: http://core.svn.wordpress.org/trunk@28475 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/wp-db.php

@@ -1314,6 +1314,10 @@ class wpdb {
 
 		$this->is_mysql = true;
 
+		/*
+		 * Deprecated in 3.9+ when using MySQLi. No equivalent
+		 * $new_link parameter exists for mysqli_* functions.
+		 */
 		$new_link = defined( 'MYSQL_NEW_LINK' ) ? MYSQL_NEW_LINK : true;
 		$client_flags = defined( 'MYSQL_CLIENT_FLAGS' ) ? MYSQL_CLIENT_FLAGS : 0;