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.

- [x] confirm that it succeeds for @raheelahmad & @jlalvarez18
- [x] confirm that it succeeds for someone who hasn’t done `brew upgrade libssh2` to get `1.4.3_1` yet
- [x] confirm that it is not linking the dylibs under `/usr/local/…` (and therefore that it can run on Macs without brew :grimacing:) **Unfortunately, we are.**
- [x] support other versions of openssl (is this sane?) ~~**Probably not sane** to try to manage upstream’s versioning carefully.~~ **Totally sane**, just change the library search path to `/usr/local/opt/openssl/lib` like brew told me to
- [x] figure out how to make it prefer `libssl.a` and `libcrypto.a` to their dylib counterparts
- [x] and then do that

Fixes #352.