§ 18-c. Sick leave for per diem employees. A per diem officer or employee holding a position in the competitive, non-competitive or labor class of the civil service of the state whose compensation or salary is on a per diem basis may be granted a sick leave with pay of not more than thirty days in each year by the head of the department, bureau, commission, agency, board or council of said state. The provisions of this section shall not apply to any employee who is subject to the rules governing sick leave, vacation and other time allowances adopted by the state civil service commission pursuant to law.