c++: address -Wweak-vtables warnings

Reported by Derek Clegg
http://lists.gnu.org/archive/html/bug-bison/2019-01/msg00021.html

    aux/parser-internal.h:429:12: error: 'syntax_error' has no out-of-line virtual
         method definitions; its vtable will be emitted in every translation unit
         [-Werror,-Wweak-vtables]
       struct syntax_error : std::runtime_error

To avoid this warning, we need syntax_error to have a virtual function
defined in a compilation unit.  Let it be the destructor.  To comply
with C++98, this dtor should be 'throw()'.  Merely making YY_NOEXCEPT
be 'throw()' in C++98 triggers
errors (http://lists.gnu.org/archive/html/bug-bison/2019-01/msg00022.html),
so let's introduce YY_NOTHROW and flag only ~syntax_error with it.

Also, since we now have an explicit dtor, we need to provide an copy
ctor.

* configure.ac (warn_cxx): Add -Wweak-vtables.
* data/skeletons/c++.m4 (YY_NOTHROW): New.
(syntax_error): Declare the dtor, and define the copy ctor.
* data/skeletons/glr.cc, data/skeletons/lalr1.cc (~syntax_error):
Define.

data/skeletons/c++.m4

@@ -77,8 +77,10 @@ m4_define([b4_cxx_portability],
 // Support noexcept when possible.
 #if 201103L <= YY_CPLUSPLUS
 # define YY_NOEXCEPT noexcept
+# define YY_NOTHROW
 #else
 # define YY_NOEXCEPT
+# define YY_NOTHROW throw ()
 #endif
 
 // Support noexcept when possible.
@@ -217,7 +219,14 @@ m4_define([b4_public_types_declare],
       syntax_error (]b4_locations_if([const location_type& l, ])[const std::string& m)
         : std::runtime_error (m)]b4_locations_if([
         , location (l)])[
-      {}]b4_locations_if([
+      {}
+
+      syntax_error (const syntax_error& s)
+        : std::runtime_error (s.what ())]b4_locations_if([
+        , location (s.location)])[
+      {}
+
+      ~syntax_error () YY_NOEXCEPT YY_NOTHROW;]b4_locations_if([
 
       location_type location;])[
     };