[OPAM] allows the use of the package alt-ergo-free as an alternative to alt-ergo

opam/opam

@@ -92,7 +92,7 @@ depends: [
   ( ( "lablgtk" { >= "2.18.2" } & "conf-gnomecanvas" )
   | ( "lablgtk3" { >= "3.0.beta4" } & "lablgtk3-sourceview3"))
   "conf-gtksourceview"
-  "alt-ergo"
+  ( "alt-ergo-free" | "alt-ergo" )
   "conf-graphviz" { post }
 ]