[ng] build: use more GNU make features in our Makefile.am

[ng] build: use more GNU make features in our Makefile.am

Mostly to show that we can do so without upsetting Automake-NG.

* Makefile.am (do_subst): Remove some code duplication through the use of
the $(foreach) builtin, and take advantage of GNU make automatic variables
to require less boilerplate in the callers.
* All its callers: Adjusted.
(EXTRA_DIST): Remove some code duplication through the use of and the
$(addprefix) builtin.  Since we are at it, remove some erroneously
duplicated entries (probably resulting from a botched merge).
(bin_SCRIPTS): Rewrite in function of $(AUTOMAKESOURCES), using the
$(basename) builtin.
(man1_MANS): Split contents into ...
(versioned_mans, unversioned_mans): ... these two variables.
(doc/aclocal-$(APIVERSION).1, doc/automake-$(APIVERSION).1): Rewrite
common rules using GNU make static pattern rules.
(automake, aclocal): Likewise.
(installcheck-executable-scripts): Take more advantage of GNU make
builtins in the recipe.
(install-data-hook): Likewise.
(doc/aclocal.1, doc/automake.1): Likewise.  Consequently ...
(update_mans): ... remove this variable, now unneeded.
($(top_srcdir)/m4/amversion.m4): Renamed ...
($(srcdir)/m4/amversion.m4): ... to this.  Since we are in a non-recursive
setup, $(srcdir) and $(top_srcdir) are the same; and according to the
comments, the use of $(top_srcdir) in this target was only needed to avoid
confusing some non-GNU make implementation.
($(srcdir)/autodeps.am): Fix a typo in the target name, renaming it ...
($(srcdir)/testsuite-autodeps.am): ... to this.  Use GNU make automatic
variables to avoid some verbosity in the recipe.  And fix a typo in
comments while we are at it.

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