Extract calc++ from the documentation.

* doc/bison.texinfo (Calc++): Add the extraction marks.
* examples/extexi: New, from the aborted GNU Programming 2E.
Separate the different paragraph of a file with empty lines.
* examples/Makefile: Use it to extract the whole calc++ example.

examples/calc++/Makefile

@@ -13,3 +13,23 @@ clean:
 	      calc++-parser.cc calc++-parser.hh \
 	      calc++-scanner.cc \
 	      calc++
+
+## ------------ ##
+## Extracting.  ##
+## ------------ ##
+
+EXTRACTED = \
+calc++-driver.hh calc++-driver.cc \
+calc++-parser.yy \
+calc++-scanner.ll \
+calc++.cc
+
+doc = ../../doc/bison.texinfo
+extexi = gawk -f ../extexi
+
+RECURSIVE_TARGETS += extract
+
+$(EXTRACTED): $(doc) ../extexi
+	$(extexi) $(doc) -- $(EXTRACTED)
+
+extract extract-am: $(EXTRACTED)