Correct bind OpenSSL configure option

Before:

```
==> ./configure --prefix=/usr/local/Cellar/bind/9.10.1 --enable-threads --enable-ipv6 --with-ssl-dir=/usr/local/opt/openssl
checking for sysconf... yes
checking for libtool... no
checking for Source Identity Token support... no
checking for OpenSSL library... configure: error: OpenSSL was not found in any of /usr /usr/local /usr/local/ssl /usr/pkg /usr/sfw; use --with-openssl=/path
If you don't want OpenSSL, use --without-openssl
```

No idea if this was caused by:

* actual change in the configure options, unlikely
* migration to 10.10 moved system OpenSSL, wasn't actually linking to brewed OpenSSL
* no bottle for 10.10 caused first build, wasn't actually linking to brewed OpenSSL but BuildBot was succeeding somehow