maint: remove most recursion in automake's own build system

maint: remove most recursion in automake's own 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.

With this change, the use of recursion in Automake's own build
system is eliminated *but for the testsuite*.  Converting that
is a little more tricky, and better left for a later change (or
series of changes).

See also commit v1.11-769-gfeeb7f6 of 28-03-2012.

* contrib/Makefile.am, lib/Makefile.am, m4/Makefile.am,
doc/Makefile.am:  Removed, their content merged ...
* Makefile.am: ... here, with required adjustments and few
other "opportunistic" changes.
(SUBDIRS): Update.
* configure.ac (AC_CONFIG_FILES): Likewise.
* .gitignore: Likewise.

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