maint: targets and recipes to simplify testing on real-world packages

maint: targets and recipes to simplify testing on real-world packages

We introduce a new section in the maintainer-specific makefile that
contains recipes to test the build system of some well-known GNU
packages with the current development version of Automake.  Not the
cleanest way to do so, but good enough for the moment.  We'll revisit
the matter after the 1.13.2 release (which we now hope will happen
soon).

* maint.mk (git-sv-host): New.
(SV_GIT_CF, SV_GIT_AC, SV_GIT_GL): Use it to reduce code duplication.
(ALL_PACKAGES, FEW_PACKAGES): New, lists of GNU packages to try out.
(ttp-check, ttp-check-all): New targets, do the checking with said
packages.
(ttp): New, alias for 'ttp-check'.
(ttp-all): New, alias for 'ttp-check-all'.

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