include: fix bug in handling of user-defined makefile fragments generation

include: fix bug in handling of user-defined makefile fragments generation

If a user defined one single Makefile fragment to be included (via Automake
includes) in his main Makefile.am, and gave a rule to generate that file
from other data, Automake used to spuriously complain about with something
like "overrides Automake target '$(srcdir)/foo.am".  This change remove that
spurious error (via a simple hack rather than a systematic change, but oh
well).

* lib/am/configure.am (%MAKEFILE-IN-DEPS%) [?HAVE-MAKEFILE-IN-DEPS?]: Add
a trailing "$(am__empty)" to the list of targets, which is enough to trick
Automake into not complaining about "duplicated targets" in case the
'%MAKEFILE-IN-DEPS%' list expands to a single target that is also declared
in some user-defined rule.
* t/list-of-tests.mk (XFAIL_TESTS): Remove now-passing test
't/am-include-only-one-generated-fragment.sh'.
* NEWS: Update.

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