cairo: make lablgtk an optional dep

Lablgtk is an optional dep for cairo:
```
[cochinois:~/tmp/cairo-0.4.2 12:44]$ocaml setup.ml -configure -help
configure options:
  --override var+val   Override any configuration variable.
  --enable-lablgtk2    Whether to connect Cairo and lablgtk2. [default: disabled]
  --disable-lablgtk2   Whether to connect Cairo and lablgtk2. [default: disabled]
...
```