glr2.cc: example: use objects (not pointers) to represent the AST

Currently we are using pointers.  The whole point of
glr2.cc (vs. glr.cc) is precisely to allow genuine C++ objects to be
semantic values.  Let's make that work.

* data/skeletons/glr2.cc (glr_state::glr_state): Be sure to initialize
yysval.
(glr_state): Add copy-ctor, assignment and dtor.
(glr_state::copyFrom): Be sure to initialize the destination if it was
not.
(glr_state::~glr_state): Destroy the semantic value.
* examples/c++/glr/ast.hh: Rewrite so that we use genuine objects,
rather than a traditional OOP hierarchy that requires to deal with
pointers.
With help from Bruno Belanyi <bruno.belanyi@epita.fr>.
* examples/c++/glr/c++-types.yy: Remove memory management.
Use true objects.
(main): Don't reach yydebug directly.

* examples/c++/glr/local.mk: We need C++11.

examples/c++/local.mk

@@ -30,7 +30,7 @@ simple_extracted = %D%/simple.yy
 simple_sources = $(simple_extracted)
 extracted += $(simple_extracted)
 
-if ENABLE_CXX14
+if ENABLE_CXX11
   check_PROGRAMS += %D%/simple
   nodist_%C%_simple_SOURCES = $(simple_sources)