Fix test_cpp11_lambda for gcc 4.8
Fix test_cpp11_lambda for gcc 4.8

* tests/test_cpp11_lambda.cc: Remove an unneccesary std::ref().
Show that std::bind() can be assigned to a slot, if it's combined with
std::function.

Distrotech/libsigcxx2,Mixaill/cmake.libsigcpp,Distrotech/libsigcxx2,Mixaill/cmake.libsigcpp,Distrotech/libsigcxx2,Mixaill/cmake.libsigcpp