(d) Bonds authorized under this section constitute a general obligation of the state. While any of the bonds or interest on the bonds is outstanding and unpaid, there is appropriated out of the first money coming into the treasury in each fiscal year, not otherwise appropriated by this constitution, the amount sufficient to pay the principal of and interest on the bonds that mature or become due during the fiscal year, less any amounts in the interest and sinking accounts at the close of the preceding fiscal year that are pledged to payment of the bonds or interest.