doc: an introductory example for C++

Suggested by Victor Khomenko.
http://lists.gnu.org/archive/html/bug-bison/2018-08/msg00037.html

* doc/bison.texi (A Simple C++ Example): New.
* examples/c++/local.mk, examples/c++/simple.test: New.
Extract, check, and install this new example.
* examples/local.mk: Adjust.
* examples/test: Adjust to the case where the dirname differs
from the test name.

examples/c++/local.mk

@@ -0,0 +1,45 @@
+## Copyright (C) 2018 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/>.
+
+## -------------------- ##
+## Building & testing.  ##
+## -------------------- ##
+
+BUILT_SOURCES += $(simple_sources)
+CLEANFILES +=  %D%/simple.[ch] %D%/simple.output
+CLEANDIRS += %D%/*.dSYM
+
+simple_extracted = %D%/simple.yy
+simple_sources = $(simple_extracted)
+extracted += $(simple_extracted)
+
+if ENABLE_CXX14
+  check_PROGRAMS += %D%/simple
+  nodist_%C%_simple_SOURCES = $(simple_sources)
+
+  %C%_simple_CXXFLAGS = $(CXX11_CXXFLAGS)
+  # Don't use gnulib's system headers.
+  %C%_simple_CPPFLAGS = -I$(top_builddir)
+  dist_TESTS += %D%/simple.test
+  %D%/simple.cc: $(BISON_IN) $(dist_pkgdata_DATA)
+endif
+
+
+## ------------ ##
+## Installing.  ##
+## ------------ ##
+
+cxxdir = $(docdir)/examples/c++
+cxx_DATA = $(simple_extracted)

examples/c++/simple.test

@@ -0,0 +1,4 @@
+#! /bin/sh
+
+: >input
+run 0 "{I have three numbers for you., 1, 2, 3, And that's all!}"