mosh: use brewed openssl as default

The openssl library bundled with OS X operation system is deprecated. It is better to link against the brewed openssl library instead of the system one.

In addition the option to link against the system one is offered.
Agreed. When I try to send a request to ``http://mosh.mit.edu/mosh-1.2.4.tar.gz``, the server send a 301 response to its https url advising we should use https as default.