If any county shall own and possess any real estate suitable for a site for a memorial building, it shall be lawful for the county board or board of county commissioners, as the case may be, by two-thirds vote, to enter into a lease for a period not exceeding ninety-nine years, by which said real estate may be leased to the organization or individuals contemplating the erection of a memorial building through private donations or contributions: Provided, the amount contributed or donated by public subscriptions for such purpose shall be equal to at least double the value of such real estate: Provided, further, that said lease shall provide that said building shall, when erected, be used exclusively for public or civic purposes, and that upon the expiration of said lease, or the violation of any of its terms or conditions, the improvements thereon shall revert to and become the property of said county.