allow/prefer a system OpenSSL

FreeBSD (and the other BSDs) bundle OpenSSL into the base system.  They may not have it installed as a package.  Attempting to configure Mosh on such a system will fail.  This builds fine on the Linux systems I have available but it might go wrong someplace else.

There are other portability fixes in the performance branch which I will be pushing shortly, I didn't fully disentangle all my commits.