Clarify error message for macOS users.

            msg.push_str(

                "

It looks like you're compiling on macOS, where the system contains a version of

OpenSSL 0.9.8. This crate no longer supports OpenSSL 0.9.8.

openssl-sys crate build failed: no supported version of OpenSSL found.

As a consumer of this crate, you can fix this error by using Homebrew to

install the `openssl` package, or as a maintainer you can use the openssl-sys

0.7 crate for support with OpenSSL 0.9.8.

Unfortunately though the compile cannot continue, so aborting.

Ways to fix it:

- Use the `vendored` feature of openssl-sys crate to build OpenSSL from source.

- Use Homebrew to install the `openssl` package.

- Downgrade to openssl-sys 0.7 crate to use the old OpenSSL 0.9.8 version

  provided by macOS.

",

            );