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.
it is a standard procedure (unix-style)  i.e. ``./configure``, ``make``, ``make install``. So I thought it was pretty nature to separate this into two steps. 

Now i have removed the duplicated ``system "make"`` line.