maint: move maintainer make rules in maint.mk

maint: move maintainer make rules in maint.mk

This is done for two reasons:

  1. It will enable us to take advantage of more GNU make
     features in our maintainer-specific recipes, if we
     want to (and we probably will).

  2. The Makefile.am won't have to be modified each time a
     maintainer rule needs to be changed or tweaked; this
     will prevent useless remaking of (in particular) the
     't/testsuite-part.am' file.

* maint.mk: New, several recipes, rules and variables for
maintainer-specific tasks, extracted (with some tweaks)
from ...
* Makefile.am: ... this file.
(EXTRA_DIST): Add 'maint.mk'.
* GNUmakefile: Include 'maint.mk' after 'Makefile'.

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