bootstrap: add convenience make target

bootstrap: add convenience make target

Dependencies in the Automake build system are not completely specified
(see for example the commit log of recent commit 'v1.12-10-gab14841',
"build: avoid too greedy rebuilds in the testsuite").  In fact, some of
them cannot even be; for example, Makefile is generated at configure
time from Makefile.in, which should be regenerated by our bleeding-edge
automake script, which is generated by out Makefile -- specifying the
complete chain of dependencies here would bring to a circular dependency
issue.

For this reason, before testing or deploying a change, we are often
forced to perform a full re-bootstrap of the Automake package, to ensure
all our files are actually up-to-date.  Until now, this has to be done
manually, thus causing wasted keystrokes and more possibilities of error.

With this change, we introduce a new 'bootstrap' make target to
automatize all the (easy) steps of this re-bootstrapping (plus some
minor bells & whistles since we are at it).

* GNUmakefile: Rewrite to allow an easy bootstrapping and clean rebuild
of the whole package, in particular with the help of ...
(bootstrap): ...  this new target.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>