If the will is contested, all the subscribing witnesses who are present in the county and who are of sound mind must be produced and examined, or the death, absence or incapacity of any of them must be satisfactorily shown to the court. If none of the subscribing witnesses resides in the county, and the evidence of none of them can be produced, the court may admit the evidence of other witnesses to prove the due execution of the will and, as evidence of the execution, it may admit proof of the handwriting of the testator and of any of the subscribing witnesses.