C99 compatiblity fixes
    
    Apply upstream patches from the portable OpenSSH project to fix
    C99 compatibility issues in the configure script.
    
    For the PAM agent integration, apply a custom downstream fix,
    as the proposed upstream changes have not been merged yet.
    
    Related to:
    
      <https://fedoraproject.org/wiki/Changes/PortingToModernC>
      <https://fedoraproject.org/wiki/Toolchain/PortingToModernC>