package: install the examples

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.