[reference-config] prefer lablgtk3 over lablgtk2

reference-configuration.md

 The following set of packages is known to be a working configuration for
 compiling Frama-C+dev, on a machine with gcc <= 9[^gcc-10]
 
+NB: frama-c-gui can be compiled against either lablgtk3 or lablgtk2. However, gtk2 itself
+is deprecated on most distributions, and it can be difficult to install the appropriate
+support libraries (notably gtksourceview). lablgtk3 should be preferred.
+
 - OCaml 4.08.1
 - alt-ergo.2.2.0 (for wp, optional)
 - apron.v0.9.12 (for eva, optional)
-- lablgtk.2.18.11 | lablgtk3.3.1.1 + lablgtk3-sourceview3.3.1.1
+- lablgtk3.3.1.1 + lablgtk3-sourceview3.3.1.1 | lablgtk.2.18.11
 - mlgmpidl.1.2.12 (for eva, optional)
 - ocamlfind.1.8.1
 - ocamlgraph.1.8.8
@@ -16,5 +20,5 @@ compiling Frama-C+dev, on a machine with gcc <= 9[^gcc-10]
 
 [^gcc-10]: As mentioned in this [OCaml PR](https://github.com/ocaml/ocaml/issues/9144)
 gcc 10 changed its default linking conventions to make them more stringent,
-resulting in various linking issues. In particular, only OCaml 4.10 can be
+resulting in various linking issues. In particular, only OCaml >= 4.10 can be
 linked against gcc-10.