C++: fix -Wdeprecated warnings

For instance on test 99:

    In file included from @@.cc:56:
    @@.hh:409:26: error: definition of implicit copy constructor for
                         'stack_symbol_type' is deprecated because it
                         has a user-declared copy assignment operator
                         [-Werror,-Wdeprecated]
          stack_symbol_type& operator= (const stack_symbol_type& that);
                         ^

Reported by Derek Clegg.
https://lists.gnu.org/archive/html/bison-patches/2018-05/msg00036.html

* configure.ac (warn_tests): Add -Wdeprecated.
* data/lalr1.cc (stack_symbol_type): Add an explicit copy ctor.
We cannot rely on the explicit default implementation (`= default`)
as we support C++ 98.

THANKS

@@ -40,6 +40,7 @@ Daniel Hagerty            hag@gnu.org
 David J. MacKenzie        djm@gnu.org
 David Kastrup             dak@gnu.org
 Dennis Clarke             dclarke@blastwave.org
+Derek Clegg               derek@me.com
 Derek M. Jones            derek@knosof.co.uk
 Di-an Jan                 dianj@freeshell.org
 Dick Streefland           dick.streefland@altium.nl

configure.ac

@@ -94,7 +94,7 @@ if test "$enable_gcc_warnings" = yes; then
   # -Wno-keyword-macro: We use the "#define private public" dirty
   # trick in the test suite to check some private implementation
   # details for lalr1.cc.
-  warn_tests='-Wundef -pedantic -Wsign-compare -fno-color-diagnostics
+  warn_tests='-Wundef -pedantic -Wdeprecated -Wsign-compare -fno-color-diagnostics
     -Wno-keyword-macro'
 
   AC_LANG_PUSH([C])

data/lalr1.cc

@@ -314,6 +314,8 @@ b4_location_define])])[
       typedef basic_symbol<by_state> super_type;
       /// Construct an empty symbol.
       stack_symbol_type ();
+      /// Copy construct.
+      stack_symbol_type (const stack_symbol_type& that);
       /// Steal the contents from \a sym to build this.
       stack_symbol_type (state_type s, symbol_type& sym);
       /// Assignment, needed by push_back.
@@ -573,6 +575,13 @@ m4_if(b4_prefix, [yy], [],
   ]b4_parser_class_name[::stack_symbol_type::stack_symbol_type ()
   {}
 
+  ]b4_parser_class_name[::stack_symbol_type::stack_symbol_type (const stack_symbol_type& that)
+    : super_type (that.state]b4_locations_if([, that.location])[)
+  {
+    ]b4_variant_if([b4_symbol_variant([that.type_get ()],
+                                      [value], [copy], [that.value])],
+                   [[value = that.value;]])[
+  }
 
   ]b4_parser_class_name[::stack_symbol_type::stack_symbol_type (state_type s, symbol_type& that)
     : super_type (s]b4_locations_if([, that.location])[)