[bug] Darwin not considered a UNIX system?

This package is a dependency of gravitational/teleport, which instructs clients to set the `SSL_CERT_FILE` and `SSL_CERT_DIR` to override the RootCA.

The docs [here](https://github.com/google/certificate-transparency-go/blob/master/x509/x509.go#L7) say that on UNIX systems these flags work.

However the build constraints [here](https://github.com/google/certificate-transparency-go/blob/master/x509/root_unix.go#L5) do not include `darwin` which is Unix-like. Is this intentional?

Start with x509/root_unix.go and compare its build constraints with the UNIX behavior described in x509/x509.go. Determine whether Darwin should be included, then verify that the documented SSL_CERT_FILE and SSL_CERT_DIR behavior matches the resulting platform coverage.

Written by the indexing model from the issue text.