[WIP] Fix linking against new libssh2 formula

:rotating_light: :warning: **Warning:** This is not ready to merge until we have confirmation that it doesn’t break the build for folks who haven’t updated brew yet, and that it actually works correctly in deployed projects linking against `ObjectiveGit.framework`. :rotating_light: :warning:

This is intended to fix linking Objective-Git against the libssh2 that `brew` gives us, which apparently can no longer be linked against system openssl.