add c++11 option to libsigc++ (updated)

See https://git.gnome.org/browse/libsigc++2/commit/tests/test_cpp11_lambda.cc?id=cd600a31fbf8e76e25f4be4c10c0645f090a9b80 for the upstream patch.
 But we don't used it because patch will fail with it in the first hunk (maybe because the line offset exceeds threshold).