The board of supervisors of any county are hereby authorized to levy a tax of not to exceed 2 mills on the assessed valuation of said county each year for a period of not to exceed 10 years, for the purpose of creating a sinking fund to be used for the purchase of real estate for sites for, and the construction or repair of public buildings; provided the proposition of levying such tax to create such sinking fund shall be submitted to the electors of the county and approved by a majority of those voting thereon in the manner provided in this act.