maint.mk: allow announcement for non-gnulib project

maint.mk: allow announcement for non-gnulib project

Currently, autoconf does not use a gnulib submodule; this
tripped up the 'make beta' with:

announce-gen: when specifying gnulib as a tool, you must also specify
--gnulib-version=V, where V is the result of running git describe
in the gnulib source directory.
Try 'announce-gen --help' for more information.

* maint.mk (announcement): Skip gnulib version if not used.

Signed-off-by: Eric Blake <eblake@redhat.com>