glr2.cc: add an example

Currently this example crashes on input such as "T (x) + y;".
The same example with glr.c works properly.

* examples/c++/glr/Makefile, examples/c++/glr/README.md,
* examples/c++/glr/c++-types.test, examples/c++/glr/c++-types.yy,
* examples/c++/glr/local.mk, examples/c++/local.mk: New.
Based on examples/c/glr/c++-types.y.

examples/c++/glr/local.mk

@@ -0,0 +1,46 @@
+## Copyright (C) 2020 Free Software Foundation, Inc.
+##
+## This program is free software: you can redistribute it and/or modify
+## it under the terms of the GNU General Public License as published by
+## the Free Software Foundation, either version 3 of the License, or
+## (at your option) any later version.
+##
+## This program is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+glrxxdir = $(docdir)/%D%
+
+## ----------- ##
+## c++-types.  ##
+## ----------- ##
+
+%D%/c++-types.stamp: $(dependencies)
+$(nodist_%C%_c___types_SOURCES): %D%/c++-types.stamp
+	@test -f $@ || rm -f %D%/c++-types.stamp
+	@test -f $@ || $(MAKE) $(AM_MAKEFLAGS) %D%/c++-types.stamp
+CLEANFILES += \
+  $(nodist_%C%_c___types_SOURCES) %D%/c++-types.stamp
+CLEANDIRS += %D%/*.dSYM
+
+## -------------------- ##
+## Building & testing.  ##
+## -------------------- ##
+
+# Avoid using BUILT_SOURCES which is too global.
+$(%C%_c___types_OBJECTS): $(cxx_types_sources_generated)
+
+if ENABLE_CXX
+  check_PROGRAMS += %D%/c++-types
+  nodist_%C%_c___types_SOURCES =		\
+    %D%/c++-types.cc				\
+    %D%/c++-types.hh
+  # Don't use gnulib's system headers.
+  %C%_c___types_CPPFLAGS = -I$(top_srcdir)/%D% -I$(top_builddir)/%D%
+  TESTS += %D%/c++-types.test
+endif ENABLE_CXX
+EXTRA_DIST += %D%/c++-types.yy %D%/c++-types.test