Except as specifically provided in this chapter, no other provision of law which provides wholly or partly at the expense of this state for pensions or retirement benefits for teachers in the state, their surviving spouses, or their dependents shall apply to members or beneficiaries of this retirement system, their surviving spouses, or their dependents, provided that nothing in this chapter shall prevent the governing boards of the public school systems of the state and the Board of Regents of the University System of Georgia from making provision for supplementing the retirement and pension allowances of members of the retirement system.