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)

data/glr.cc

@@ -253,6 +253,7 @@ m4_define([b4_shared_declarations],
 [m4_pushdef([b4_parse_param], m4_defn([b4_parse_param_orig]))dnl
 b4_percent_code_get([[requires]])[
 
+#include <cstdlib>  // abort
 #include <stdexcept>
 #include <string>
 #include <iostream>]b4_defines_if([

data/lalr1.cc

@@ -146,6 +146,7 @@ b4_variant_if([m4_include(b4_pkgdatadir/[variant.hh])])
 m4_define([b4_shared_declarations],
 [b4_percent_code_get([[requires]])[
 ]b4_parse_assert_if([# include <cassert>])[
+# include <cstdlib>  // abort
 # include <vector>
 # include <iostream>
 # include <stdexcept>

data/variant.hh

@@ -203,7 +203,7 @@ m4_define([b4_variant_define],
     }
 
     /// Prohibit blind copies.
-    //  private:
+    private:
     self_type& operator=(const self_type&)
     {
       abort ();