MYSQL_SECURE_AUTH has been removed in MySQL 8.0.3 RC (#892)

https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-3.html#mysqld-8-0-3-capi
> The deprecated secure_auth system variable and --secure-auth client option have been removed.
> The MYSQL_SECURE_AUTH option for the mysql_options() C API function was removed.

ext/mysql2/client.c

@@ -883,10 +883,12 @@ static VALUE _mysql_client_options(VALUE self, int opt, VALUE value) {
       retval = &boolval;
       break;
 
+#if defined(MYSQL_SECURE_AUTH)
     case MYSQL_SECURE_AUTH:
       boolval = (value == Qfalse ? 0 : 1);
       retval = &boolval;
       break;
+#endif
 
     case MYSQL_READ_DEFAULT_FILE:
       charval = (const char *)StringValueCStr(value);
@@ -1312,7 +1314,10 @@ static VALUE set_ssl_options(VALUE self, VALUE key, VALUE cert, VALUE ca, VALUE
 }
 
 static VALUE set_secure_auth(VALUE self, VALUE value) {
+/* This option was deprecated in MySQL 5.x and removed in MySQL 8.0 */
+#if defined(MYSQL_SECURE_AUTH)
   return _mysql_client_options(self, MYSQL_SECURE_AUTH, value);
+#endif
 }
 
 static VALUE set_read_default_file(VALUE self, VALUE value) {