freetds.rb: Add option --enable-msdblib so that users can get SQL Server compatibility without hacking the formula

This enables a `--enable-msdblib` option:

```
marca@marca-mac:~/dev/git-repos/pymssql$ brew install freetds --enable-msdblib
…
marca@marca-mac:/usr/local$ tsql -C | grep 'db-lib'
     MS db-lib source compatibility: yes
```

If you Google for "freetds brew msdblib" or such, you will find people that recommend installing FreeTDS by editing `freetds.rb` to add the `--enable-msdblib` option. We should just make it an option so people don't need to hack the formula.