Make sub-directories visible the automake's "dist*" targets

Make sub-directories visible the automake's "dist*" targets

We have a few directories with source codes that we tell configure to
prepare, but we do not actually want them built during normal operations
(tests and examples only).

However, there are some special targets brought by automake which still
need to see them, so this patch adds these directories to the list, but
only for these rules, we keep them unvisited by the normal build process.

The wanted side effect of this is that now "make distcheck" works untill
the end as expectable.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>