Include config.h in all files using LIBDNS_VERSION_MAJOR
    
    Did not know compiler replaces unknown symbols with 0. It made the check
    silently evaluate to false on every BIND version, regardless detected
    libdns version number. Generated code failing runtime with BIND 9.16.
    
    Signed-off-by: Petr Menšík <pemensik@redhat.com>