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/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([