build: break up monolithic Makefile.am in subdir-specific fragments

build: break up monolithic Makefile.am in subdir-specific fragments

This is convenient to do, now that we have improved "relative directory"
support with the '%reladir%' (a.k.a. '%D%') and '%canon_reladir%' (a.k.a.
'%C%') Automake-time substitutions for included makefile fragments.

This move also satisfy our philosophy of using new Automake features in
our own build system, as a way of facilitating early discovery of possible
bugs or interface warts.

* Makefile.am: Break up ...
* doc/Makefile.inc, lib/Automake/Makefile.inc, lib/Makefile.inc,
lib/am/Makefile.inc, m4/Makefile.inc, t/Makefile.inc): ... in this
new included fragments.  Adjust as needed, and make deliberate use
of the '%D%' substitution.
* contrib/t/local.am: Rename ...
* contrib/t/Makefile.inc: ... like this.

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