[cpp] C++11 examples are not compiled by default

These C++11 examples are not compiled by default:

• multithreaded_client
• multithreaded_client_flow_control
• scheduled_send
• service_bus

cmake is checking CPP11 availability, but it looks that it is not enough and then it does not compile C++11 examples.

But I am able to compile these examples by:
g++ <example_name>.cpp -o <example_name> -lqpid-proton-cpp -std=c++11