maint: de-recurse the handling of examples

maint: de-recurse the handling of examples

The directory was still using a local Makefile.am because it provides
"scoped" Make variables: these examples are not meant to use the same
CPPFLAGS etc.  If we were to use the same -I set, we'd pick up
gnulib's stdio.h for instance, which we do not want for these simple
examples.

Yet, as a result, the dependencies are less accurate, there is code
duplication, etc.  This is especially perceptible when trying to
extract more examples from the documentation, as will be done in
forthcoming changes.

In order to make the tuning of CPPFLAGS easier, discard the predefined
-I from Automake.

	* examples/calc++/Makefile.am: Rename as...
	* examples/calc++/local.mk: this.
	Adjust the paths which are now rooted in top_srcdir/top_builddir.
	Handle BISON_CXX_WORKS here, instead of the too crude previous
	approach that completely discarded the whole directory.
	($(BISON)): Remove now useless bouncing recipe.
	(calc___CPPFLAGS): New.
	Stay away from -Ilib.
	* Makefile.am, configure.ac, examples/local.mk,
	* examples/calc++/test: Adjust.

	* configure.ac: Pass nostdinc to Automake.
	* src/local.mk, lib/local.mk (AM_CPPFLAGS): Move to...
	* Makefile.am: here.

	* src/local.mk, examples/calc++/Makefile.am (BISON, BISON_IN): Factor
	to...
	* Makefile.am: here.
	* tests/local.mk: Use it.