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.

configure.ac

@@ -100,7 +100,7 @@ if test "$enable_gcc_warnings" = yes; then
     -Wpointer-arith -Wshadow
     -Wwrite-strings'
   warn_c='-Wbad-function-cast -Wstrict-prototypes'
-  warn_cxx='-Wextra-semi -Wnoexcept -Wundefined-func-template'
+  warn_cxx='-Wextra-semi -Wnoexcept -Wundefined-func-template -Wweak-vtables'
   # Warnings for the test suite only.
   #
   # -fno-color-diagnostics: Clang's use of colors in the error