maint: fix example extraction issues.

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

Basically, revert commit 609b3d8096,
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.

examples/extexi

@@ -29,7 +29,7 @@ BEGIN {
     if (ARGV[argc] == "--")
       break;
   for (i = argc + 1; i < ARGC; ++i)
-    file_wanted[ARGV[i]] = 1;
+    file_wanted[basename(ARGV[i])] = ARGV[i];
   ARGC = argc;
 }
 
@@ -43,13 +43,10 @@ BEGIN {
 }
 
 /^@comment file: / {
-  if (!file_wanted[$3])
-    message("ignoring " $3);
+  if (file = file_wanted[$3])
+    message(" GEN " file);
   else
-    {
-      message("extracting " $3);
-      file = $3;
-    }
+    message("SKIP " $3);
 }
 
 /^@(small)?example$/, /^@end (small)?example$/ {
@@ -125,6 +122,11 @@ function normalize(contents,    i, lines, n, line, res) {
 }
 
 
+function basename(name,     a, n) {
+  n = split (name, a, "/");
+  return a[n];
+}
+
 function message(msg) {
   if (! message_printed[msg])
     {