build-sys: Add --with-osid
    
    This new configure flag allows to specify a string ID (eg fedora22,
    ubuntu10.04, ..) identifying the OS this remote-viewer build will be
    for. This will be used in combination with the new 'versions' field in
    .vv files in order to make it possible for the creator of the .vv file
    to specify which version it expects for the various OSes which may
    connect.