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/test

@@ -15,7 +15,8 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-me=`basename $1 .test`
+me=$(basename "$1" .test)
+medir=$(basename "$(dirname "$1")")
 
 # Number of the current test.
 number=1
@@ -27,7 +28,7 @@ exit=true
 cwd=`pwd`
 
 # The exercised program.
-for p in $cwd/examples/$me/$me $cwd/examples/$me
+for p in "$cwd/examples/$medir/$me" "$cwd/examples/$me"
 do
     if test -x "$p"; then
         prog=$p