minimum needed Automake version

[Bruno Haible]

Hi,

Since 2020-05-10, the continuous integration builds are failure during
"make distcheck" like this:

Making distclean in libpoke
make[2]: Entering directory 
'/builds/gnu-poke/ci-distcheck/poke/poke-0.1-20200516-beta/_build/sub/libpoke'
test -z "" || rm -f 
test -z "libpoke.la" || rm -f libpoke.la
rm -f ./so_locations
rm -rf .libs _libs
rm -rf ../common/.libs ../common/_libs
test -z "libpvmjitter.la" || rm -f libpvmjitter.la
rm -f ./so_locations
rm -f *.o
rm -f ../common/*.o
rm -f ../common/*.lo
test -z "pkl-tab.output" || rm -f pkl-tab.output
rm -f *.lo
rm -f *.tab.c
test -z "" || rm -f 
test . = "../../../libpoke" || test -z "" || rm -f 
rm -f ../common/.deps/.dirstamp
rm -f ../common/.dirstamp
test -z "" || rm -f 
rm -f TAGS ID GTAGS GRTAGS GSYMS GPATH tags
rm -rf ../common/.deps ./.deps
rm -f Makefile
make[2]: Leaving directory 
'/builds/gnu-poke/ci-distcheck/poke/poke-0.1-20200516-beta/_build/sub/libpoke'
Making distclean in poke
make[2]: Entering directory 
'/builds/gnu-poke/ci-distcheck/poke/poke-0.1-20200516-beta/_build/sub/poke'
Makefile:1933: ../common/.deps/poke-pk-utils.Po: No such file or directory
make[2]: *** No rule to make target '../common/.deps/poke-pk-utils.Po'.  Stop.

This failure can be observed in a build environment with Automake 1.15.1,
but not in a build environment with Automake 1.16.2.

What happens is that in the "Making distclean in libpoke" step, with
Automake 1.15.1, the file common/.deps/poke-pk-utils.Po gets removed,
and the "Making distclean in poke" step then falls flat on its nose
because poke/Makefile contains an 'include "../common/.deps/poke-pk-utils.Po"'
statement.

With Automake 1.16.2, the file common/.deps/poke-pk-utils.Po still
exists after the "Making distclean in libpoke" step.

Indeed, according the Automake's NEWS file, at least two bugs relating to
dependencies and subdir-object were fixed in version 1.16. This appears
to be a third one, fixed as well in version 1.16.

So, the fix is to
  - use Automake 1.16.2 in the CI - done,
  - require Automake 1.16 or newer - proposed patch attached.

Bruno

0001-build-Bump-minimum-needed-Automake-version.patch
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