Make OpenSSL support optional / dependent on rabbitmq-c

[kblaschke]

Currently, SimpleAmqpClient implicitly requires a rabbitmq-c library compiled with SSL support. In environments where the complexity of SSL support is not a requirement, developers are currently forced to link both libraries against OpenSSL. While some people may argue this is not much of an overhead and OpenSSL is readily available on many platforms, it'd still be cleaner to keep SimpleAmqpClient in line with rabbitmq-c and have optional SSL support.

I would contribute the required changes, but before this, like to discuss the best way to implement it. My suggestion is:

• In CMakeLists.txt, check for the availability of amqp_ssl_socket.h. If the file exists, provide the user with an option to enable of disable SSL support, and default it to "ON".
• Split up the AmqpClient::Channel class, and move the SSL-specific create() function and the associated constructor into a derived class, e.g. SecureChannel.
• If SSL support is ON, add the SecureChannel implementation and header files to the build.

[alanxz]

This all sounds good to me. Be sure to also update the install command in CMakeLists.txt so that the SecureChannel.h conditionally gets installed.