package: install the examples

Currently, we do not install the various examples extracted from the
documentation.  Let's do it, as they are useful starting points.

* configure.ac: When --enable-gcc-warnings is set, enable ENABLE_GCC_WARNINGS.
* examples/extexi: No longer issue synclines by default.
* examples/local.mk: Except if ENABLE_GCC_WARNINGS.
* examples/calc++/local.mk, examples/mfcalc/local.mk,
* examples/rpcalc/local.mk: Install the example files.

NEWS

@@ -24,6 +24,20 @@ GNU Bison NEWS
   Now, the "previous" declaration is always the first one.
 
 
+** Documentation
+
+  Bison now installs various files in its docdir (which defaults to
+  '/usr/local/share/doc/bison'), including the three fully blown examples
+  extracted from the documentation:
+
+   - rpcalc
+     Reverse polish calculator, a simple introductory example.
+   - mfcalc
+     Multi-function Calc, a calculator with memory and functions and located
+     error messages.
+   - calc++
+     a calculator in C++ using variant support and token constructors.
+
 * Noteworthy changes in release 3.0.2 (2013-12-05) [stable]
 
 ** Bug fixes