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, 1 insertions, 1 deletions

diff --git a/pretyping/classops.ml b/pretyping/classops.ml
index 55220f44c0..d3d4201f57 100644
--- a/pretyping/classops.ml
+++ b/pretyping/classops.ml
@@ -387,7 +387,7 @@ let add_coercion_in_graph (ic,source,target) =
   end;
   let is_ambig = match !ambig_paths with [] -> false | _ -> true in
   if is_ambig && is_verbose () then
-    Feedback.msg_warning (message_ambig !ambig_paths)
+    Feedback.msg_info (message_ambig !ambig_paths)
 
 type coercion = {
   coercion_type   : coe_typ;