Support setting custom lib version suffixes.

Different distros may wish to set custom library suffixes depending on their own considerations. For example, it may be desirable to have `-qt4` and `-qt5` suffixes respectively. This PR adds this via a custom `CUSTOM_LASTFM_LIB_VERSION_SUFFIX` parameter, like this:
```
cmake -DCUSTOM_LASTFM_LIB_VERSION_SUFFIX="-qt4"
```

The default behavior isn't changed.