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/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>