No usable OpenSSL found

I get the following warnings/errors when trying to install erlang 21.0:

```
 * crypto         : No usable OpenSSL found
 * ssh            : No usable OpenSSL found
 * ssl            : No usable OpenSSL found
```

I'm on MacOS 10.13.6, updated asdf-erlang and have openssl installed via homebrew:

```
% brew ls --versions openssl
openssl 1.0.2o_1 1.0.2p 1.0.2o_2
```

Any ideas?