Activate the Sphinx doctest extension and convert howto/functional to use it.
diff --git a/Doc/Makefile b/Doc/Makefile
index e94be81..64a1b2f 100644
--- a/Doc/Makefile
+++ b/Doc/Makefile
@@ -82,6 +82,11 @@
 coverage: build
 	@echo "Coverage finished; see c.txt and python.txt in build/coverage"
 
+doctest: BUILDER = doctest
+doctest: build
+	@echo "Testing of doctests in the sources finished, look at the " \
+	      "results in build/doctest/output.txt"
+
 clean:
 	-rm -rf build/*
 	-rm -rf tools/sphinx