cmake: Link to Homebrew OpenSSL on MacOS by default
cmake: Link to Homebrew OpenSSL on MacOS by default

This should eliminate weird linker errors, plus you can always override
it if you want. We technically already do this in Travis, so may as well
make it a default behavior and save some headaches.

Fixes #385

blindsighttf2/Astron,blindsighttf2/Astron,blindsighttf2/Astron,blindsighttf2/Astron