freetds.rb: Add options --enable-msdblib and --enable-sybase-compat so that users can get SQL Server or Sybase 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.

I also added a `--enable-sybase-compat` option:

```
marca@marca-mac:/usr/local$ brew install freetds --enable-sybase-compat
...
marca@marca-mac:/usr/local$ tsql -C | grep 'Sybase'
        Sybase binary compatibility: yes
```