[wg-camlp4] Built-in attributes

[Alain Frisch]

Dear all,

In order to mark cleanly which warnings are understood in a special way 
by the compiler, I've added an ocaml.* prefix to them.   Currently, the 
only such attributes are ocaml.deprecated and ocaml.warning.

I'm planning to add (with a low priority) a new warning to report uses 
of unknown ocaml.* warnings (or known warnings applied to a wrong 
object).  This will allow to catch typos such as writing ocaml.warnings 
instead of ocaml.warning.


Alain