Allow %{ocaml-config:..} wherever variables are allowed

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/super_context.ml

@@ -100,20 +100,36 @@ let expand_macro t ~syntax_version ~var =
     Exn.code_error "expand_macro can't expand variables"
       [ "var", String_with_vars.Var.sexp_of_t var ]
 
+let expand t ~syntax_version ~var =
+  match
+    match String_with_vars.Var.destruct var with
+    | Var _ -> Left (expand_vars t ~syntax_version ~var)
+    | Macro (_, _) -> Right (expand_macro t ~syntax_version ~var)
+  with
+  | Right None
+  | Left None -> None
+  | Right (Some x) -> Some (Right x)
+  | Left (Some x) -> Some (Left x)
+
 let (expand_vars_string, expand_vars_path) =
   let expand t ~scope ~dir ?(extra_vars=String.Map.empty) s =
     String_with_vars.expand ~mode:Single ~dir s ~f:(fun var syntax_version ->
-      match expand_vars t ~syntax_version ~var with
+      match expand t ~syntax_version ~var with
       | None ->
         String.Map.find extra_vars (String_with_vars.Var.full_name var)
-      | Some v ->
+      | Some (Left v) ->
         begin match Pform.Var.to_value_no_deps_or_targets ~scope v with
         | Some _ as v -> v
         | None ->
           Loc.fail (String_with_vars.Var.loc var)
             "Variable %a is not allowed in this context"
             String_with_vars.Var.pp var
-        end)
+        end
+      | Some (Right Ocaml_config) ->
+        String.Map.find t.ocaml_config (String_with_vars.Var.name var)
+      | Some (Right _) ->
+        Loc.fail (String_with_vars.Var.loc var)
+          "This percent form isn't allowed in this position")
   in
   let expand_vars t ~scope ~dir ?extra_vars s =
     expand t ~scope ~dir ?extra_vars s

test/blackbox-tests/test-cases/macro-expand-error/run.t

@@ -4,5 +4,5 @@ inappropariate place:
   $ dune build
   Info: creating file dune-project with this contents: (lang dune 1.0)
   File "dune", line 1, characters 14-21:
-  Error: macros of the form %{name:..} cannot be expanded here
+  Error: This percent form isn't allowed in this position
   [1]

test/blackbox-tests/test-cases/ocaml-config-macro/run.t

@@ -2,6 +2,3 @@
 the values are all platform specific.
 
   $ dune build
-  File "dune", line 3, characters 41-61:
-  Error: Unknown form: %{ocaml-config:system}
-  [1]