maint: fix example extraction issues.

maint: fix example extraction issues.

calc++: don't rely on Automake to compile a C++ parser.

Basically, revert commit 609b3d8096fb0cc1fa4d908b6e1a860ced260bda,
Automake 1.11.3 is not safe enough for C++ parser.

	* examples/calc++/calc++-parser.hh: Remove.
	* examples/calc++/local.mk (examples/calc++/calc++-parser.stamp):
	New.

examples: factor the extractions into a single step

extexi had to be run in the extraction directory.  Now, it can be
given the files with expected output directory.  This allows to
use $(*_extracted) variables (before we had to list again their
members, but limited to their base names).  In turn, this also
allows fusing the extraction recipes into a single one.

Also, it is currently too hard (or requires too much duplication,
since Make wants all the occurrences of the files to be prefixed with
$(srcdir)/, which is something Automake cannot support for *_SOURCES)
to work in the source tree.  So extract, and compile scanners and parsers
in the build tree.

	* examples/extexi (basename): New.
	(BEGIN): Now "file_wanted" maps base name to extracted file name.
	* examples/calc++/local.mk, examples/mfcalc/local.mk,
	* examples/rpcalc/local.mk: Fuse extraction rules into...
	* examples/local.mk: Here.
	(extract, extracted): New.