c++: privatize variant blind copies

* data/variant.hh (variant, operator=): Make private.
* data/c++.m4 (operator=): New, to avoid needing a definition of that operator
for each class member (such as a possible variant).
* data/glr.cc, data/lalr.cc: Add the necessary include for the abort.

data/c++.m4

@@ -186,6 +186,9 @@ m4_define([b4_public_types_declare],
         [const semantic_type& v],
         b4_locations_if([const location_type& l]))[);
 
+      /// Assignment operator.
+      inline basic_symbol& operator= (const basic_symbol& other);
+
       /// Destructive move, \a s is emptied.
       inline void move (basic_symbol& s);
 
@@ -250,6 +253,14 @@ m4_define([b4_public_types_define],
     , location (l)
   {}])[
 
+  template <typename Base>
+  inline
+  ]b4_parser_class_name[::basic_symbol<Base>&
+  ]b4_parser_class_name[::basic_symbol<Base>::operator= (const basic_symbol& other)
+  {
+    abort ();
+  }
+
   template <typename Base>
   inline
   ]b4_parser_class_name[::basic_symbol<Base>::basic_symbol (const basic_symbol& other)