An indenture may include a clause relating to the bonds issued thereunder requiring the board to pay and discharge or cause to be paid and discharged all lawful claims for labor, materials and supplies or other charges which, if unpaid, might become a lien or charge upon the revenues, or any part thereof, of any project acquired, constructed or completed from the proceeds of the sale of the bonds, or upon any physical properties, or which might impair the security of the bonds.