Create thin archive and convert it afterwards
    
    While this worked fine on rawhide, the MRI script is not accepted
    on f37. Apparently + is a special character and something we end
    up trying to use libc rather than libc++.a.
    
    Avoid this by creating a thin archive first and then only using
    the MRI script to convert it into a fat archive. Weird that there
    is apparently no native way to do this.