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.
Looks good :+1:. No need to change now, but for future reference Homebrew is transitioning from single quotes to double quotes, and ``` make install ``` to ``` "make", "install" ```.