Log Message:
ocaml-lablgtk: update to 2.18.8.

In Lablgtk-2.18.8:

2019.4.25 [Jacques]
  * release new version, as 2.18.7 fails on 4.08 (boxing problem)

2019.4.12 [Jacques]
  * remove -warn-error to allow compilation on future versions of ocaml

2019.2.15 [Jacques]
  * mark Gdk.PointArray.t and Gdk.SegmentArray.t as [@@ boxed]
    (report by Andre Maroneze)