`opam` install failing for lablgtk 2.18.11 on Big Sur

[thepinetree]

After updating my environment to Big Sur 11.1, the version of lablgtk on opam fails to install, with a compilation error that seems different from #64 and https://github.com/ocaml/opam-repository/issues/14718:

❯ opam install lablgtk
The following actions will be performed:
  ∗ install lablgtk 2.18.11

<><> Gathering sources ><><><><><><><><><><><><><><><><><><><><><><><><><><>  🐫
[lablgtk.2.18.11] found in cache

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><>  🐫
[ERROR] The compilation of lablgtk failed at "PATH_TO_OPAM/.opam/opam-init/hooks/sandbox.sh build make world".

#=== ERROR while compiling lablgtk.2.18.11 ====================================#
# context     2.0.7 | macos/x86_64 | ocaml-base-compiler.4.08.0 | https://opam.ocaml.org#ec3d487f
# path        ~/.opam/4.08.0/.opam-switch/build/lablgtk.2.18.11
# command     ~/.opam/opam-init/hooks/sandbox.sh build make world
# exit-code   2
# env-file    ~/.opam/log/lablgtk-21989-d0f245.env
# output-file ~/.opam/log/lablgtk-21989-d0f245.out
### output ###
# [...]
# + ocamlc.opt -a    -o lablgtksourceview2.cma  sourceView2Enums.cmo gtkSourceView2Props.cmo ogtkSourceView2Props.cmo gtkSourceView2.cmo gSourceView2.cmo -dllib -llablgtksourceview2 -cclib -llablgtksourceview2 -ccopt -framework -ccopt Cocoa  -cclib -L/usr/local/opt/gettext/lib -cclib -L/usr/local/Cellar/glib/2.66.4_1/lib -cclib -L/usr/local/Cellar/gdk-pixbuf/2.42.2/lib -cclib -L/usr/local/Cella[...]
# ocamlc.opt   -w s-3+52 -c -thread gtkThread.mli
# ocamlc.opt   -w s-3+52 -c -thread gtkThread.ml
# ocamlmktop  -thread  -o lablgtktop unix.cma threads.cma  \
# 	   -I . lablgtk.cma lablglade.cma lablgnomecanvas.cma lablrsvg.cma lablgtksourceview2.cma gtkThread.cmo
# File "_none_", line 1:
# Error: Error on dynamically loaded library: ./dlllablgtksourceview2.so: dlopen(./dlllablgtksourceview2.so, 10): Symbol not found: _gtkosx_create_cgimage_from_pixbuf
#   Referenced from: /usr/local/opt/gtk-mac-integration/lib/libgtkmacintegration-gtk2.2.dylib
#   Expected in: flat namespace
#  in /usr/local/opt/gtk-mac-integration/lib/libgtkmacintegration-gtk2.2.dylib
# make[1]: *** [lablgtktop] Error 2
# make: *** [world] Error 2



<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><>  🐫
┌─ The following actions failed
│ λ build lablgtk 2.18.11
└─
╶─ No changes have been performed

<><> lablgtk.2.18.11 troubleshooting ><><><><><><><><><><><><><><><><><><><>  🐫
=> This package requires gtk+ 2.0 development packages installed on your system
=> To solve pkg-config issues, you may need to do
   'export PKG_CONFIG_PATH=/opt/X11/lib/pkgconfig' and retry

The packages you requested declare the following system dependencies. Please make sure they are installed before retrying:
    expat gtk

I've ensured to set PKG_CONFIG_PATH:

❯ printenv PKG_CONFIG_PATH
/opt/X11/lib/pkgconfig:/usr/local/opt/zlib/lib/pkgconfig

and observe the following brew installations:

❯ brew list --formula | grep gtk
gtk+
gtk+3
gtk-mac-integration
gtksourceview
gtksourceview4
gtksourceview@4
lablgtk
pygtk

❯ brew list --formula | grep expat
expat