maint: reduce use of recursion in automake build system

maint: reduce use of recursion in automake build system

Recursive make-based build systems tend to be slower, more fragile
and less faithful than "flat" ones.  See Peter Miller's article
"Recursive Make Considered Harmful" for more a more in-depth
discussion:

  <http://miller.emu.id.au/pmiller/books/rmch/>

While in the case of automake this isn't a big problem (given the
small size of its build systems), it still creates occasional
glitches and annoyances.  So, with this change, we start making
less use of make recursion in the Automake's own build system
(future changes will reduce it even more).

* lib/am/Makefile.am, lib/Automake/Makefile.am:  Removed, their
content merged ...
* lib/Makefile.am: ... here, with required adjustments.
* configure.ac (AC_CONFIG_FILES): Update.

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