automake 1.18.94

lib/am/distdir.am: fix a latent recursive make distcheck bug.

* lib/am/distdir.am: `make dist' no longer propagates its internal distribution-directory cleanup override to recursive makes run while constructing distdir. The override could make a nested `make distcheck' leave its distribution directory behind and then fail its distclean check.
* NEWS: mention that.
* t/dist-multiple-formats.sh: Retrofit an existing test to also work as a regression test for this commit.