A new infrastructure for warnings.

On the user side, coqtop and coqc take a list of warning names or categories
after -w. No prefix means activate the warning, a "-" prefix means deactivate
it, and "+" means turn the warning into an error. Special categories include
"all", and "default" which contains the warnings enabled by default.

We also provide a vernacular Set Warnings which takes the same flags as argument.

Note that coqc now prints warnings.

The name and category of a warning are printed with the warning itself.

On the developer side, Feedback.msg_warning is still accessible, but the
recommended way to print a warning is in two steps:

1) create it by:

let warn_my_warning =
  CWarnings.create ~name:"my-warning" ~category:"my-category"
    (fun args -> Pp.strbrk ...)

2) print it by:

warn_my_warning args

1 files changed, 7 insertions, 3 deletions

diff --git a/pretyping/glob_ops.ml b/pretyping/glob_ops.ml
index 04100c8a7..5c8060996 100644
--- a/pretyping/glob_ops.ml
+++ b/pretyping/glob_ops.ml
@@ -334,10 +334,14 @@ let glob_visible_short_qualid c =
         fold_glob_constr aux acc c
   in aux [] c
 
+let warn_variable_collision =
+  let open Pp in
+  CWarnings.create ~name:"variable-collision" ~category:"ltac"
+         (fun name ->
+          strbrk "Collision between bound variables of name " ++ pr_id name)
+
 let add_and_check_ident id set =
-  if Id.Set.mem id set then
-    Feedback.msg_warning
-      Pp.(str "Collision between bound variables of name " ++ Id.print id);
+  if Id.Set.mem id set then warn_variable_collision id;
   Id.Set.add id set
 
 let bound_glob_vars =