Trigger creation syntax error
I didn't find anything wrong with your statement, you need perhaps DELIMITERs 

DELIMITER $$
CREATE
    TRIGGER `livre_disponible` BEFORE INSERT ON `livres` 
    FOR EACH ROW 
   BEGIN
        IF NEW.nb_exemplaires=0 THEN
            SET NEW.disponible=0;
        ELSEIF NEW.nb_exemplaires&gt;=1 THEN
            SET NEW.disponible=1;
        END IF;
    END;
$$

DELIMITER ;



  Updated : you missed also a ';' ==> END IF; END;