• add an option CPP-NETLIB_ENABLE_HTTPS to enable/disable HTTPS in cmake config. By default it is turned ON (enable HTTPS) so as to preserve before-patch behavior. Turn it OFF to disable entirely SSL usage and linkage in libraries and header files.
• when SSL is disabled, the 'stream_handler' struct is replaced by a simple tcp_socket, and function calls in the files option.hpp, async_server.hpp and async_connection.hpp are adapted and compiled accordingly.

Note that sync_...hpp files don't have problems (yet) since SSL is not integrated into these. Also note that a cleaner solution might be to consider implementing two separate struct 'stream_handler' and 'stream_handler_ssl', or something similar (in order to avoid most, if not all, the current #ifdef added by the commit in async_server and async_connection).

The changes to the code are quite simple but, for the record, I compiled this with the new option ON and OFF and successfully tested it for both configurations with a slightly adapted version of the asynchronous http server "hello world" example. I tried to be thorough while searching the code for SSL-related code, but I'm not very familiar with the whole code base, so some parts may have slipped past my attention.