Convert multiple variable definition warnings to complaints.

* NEWS (2.5): Add a new entry for that change.
* doc/bison.texinfo (Decl Summary): Update %define entry.
(Bison Options): Update -D/--define/-F/--force-define entry.
* src/muscle_tab.c (muscle_percent_define_insert): Implement.
* src/muscle_tab.h (muscle_percent_define_insert): Update
comments.
* tests/input.at (`%define errors'): Update.
(`%define, --define, --force-define'): Update.

NEWS

@@ -33,6 +33,8 @@ Bison News
   These features are experimental.  More user feedback will help to
   stabilize them.
 
+** Multiple %define's for any variable is now an error not a warning.
+
 ** %define can now be invoked via the command line.
 
   Each of these command-line options