easy-tag: dependency on id3lib doesn't seem to be optional anymore

I had easy-tag 2.1.7 installed w/o id3lib. However, upgrading to 2.1.8 failed:

```
==> ./configure --disable-dependency-tracking --prefix=/usr/local/Cellar/easy-tag/2.1.8
…
checking for GTK... yes
checking for recv in -lsocket... no
checking for library containing ID3Tag_Link... no
checking the id3lib version... configure: error: could not determine id3lib version
```