Compile stubs for @check

Without this, errors in C/C++ sources would be ignored when building
@check

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

src/check_rules.ml

@@ -25,3 +25,9 @@ let add_obj_dir sctx ~obj_dir =
       (Alias.check ~dir:(Obj_dir.dir obj_dir))
       ~dyn_deps
       Path.Set.empty
+
+let add_files sctx ~dir files =
+  if (Super_context.context sctx).merlin then
+    let alias = Alias.check ~dir in
+    let files = Path.Set.of_list files in
+    Rules.Produce.Alias.add_deps alias files

src/check_rules.mli

@@ -1,3 +1,9 @@
 open Stdune
 
 val add_obj_dir : Super_context.t -> obj_dir:Path.Build.t Obj_dir.t -> unit
+
+val add_files
+  : Super_context.t
+  -> dir:Path.Build.t
+  -> Path.t list
+  -> unit

src/lib_rules.ml

@@ -251,6 +251,7 @@ let build_stubs lib ~sctx ~dir ~expander ~requires ~dir_contents
     else
       []
   in
+  Check_rules.add_files sctx ~dir lib_o_files;
   match vlib_stubs_o_files @ lib_o_files with
   | [] -> ()
   | o_files -> build_self_stubs lib ~sctx ~dir ~expander ~o_files