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/variant.hh

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