[opam] deactivate dependency on lablgtk3 with macos

opam/opam

@@ -90,7 +90,7 @@ depends: [
   "zarith"
   "conf-autoconf" { build }
   ( ( "lablgtk" { >= "2.18.2" } & "conf-gnomecanvas" )
-  | ( "lablgtk3" { >= "3.0.beta4" } & "lablgtk3-sourceview3"))
+  | ( "lablgtk3" { >= "3.0.beta4" & os!="macos" } & "lablgtk3-sourceview3" ))
   "conf-gtksourceview"
   ( "alt-ergo-free" | "alt-ergo" )
   "conf-graphviz" { post }