[OCaml < 4.05] uigtk2: Hack for lablgtk < 2.18.6
[OCaml < 4.05] uigtk2: Hack for lablgtk < 2.18.6

lablgtk Gpointer API was changed in a breaking way in 2.18.6. This hack
is necessary to have single code compile both with lablgtk < 2.18.6 and
with lablgtk >= 2.18.6; the former is requred for compatibility with
OCaml < 4.05.

This patch can be reverted after the next release or when porting to
lablgtk3.

paulp/unison