Add Thread-safe implementation of strerror()
    
    Unfortunately strerror() is not thread safe so we have to juggle with
    strerror_r() which is a can of worms as 2 incompatible implementations
    are available depending on what is defined at compile time.
    
    Try to do something sane.
    
    https://fedorahosted.org/gss-proxy/ticket/111
    
    Reviewed-by: Günther Deschner <gdeschner@redhat.com>