Broken "make distcheck" with automake 1.16

[Reuben Thomas]

Building Recode[1], which uses a recent gnulib (commit 035ce56a33 of
5th January 2022), I find that when I run "make distcheck", some
.deps/*.Plo files generated by the build are not removed by any clean
rule, leading "make distcheck" to report:

ERROR: files left in build directory after distclean:
./lib/.deps/asnprintf.Plo
./lib/.deps/calloc.Plo
./lib/.deps/canonicalize-lgpl.Plo
./lib/.deps/close.Plo
./lib/.deps/dup2.Plo
./lib/.deps/error.Plo
./lib/.deps/fcntl.Plo
./lib/.deps/free.Plo
./lib/.deps/fseterr.Plo
etc.

On investigation, the relevant object files are listed in gl_LIBOBJS
and gl_LTLIBOBJS, which is used by libgnu_la_LIBADD.

With automake 1.15, the distclean rule looks like this:

distclean: distclean-recursive
        -rm -rf ./$(DEPDIR) malloc/$(DEPDIR)

With automake 1.16, the distclean rule looks like this:

distclean: distclean-recursive
                -rm -f ./$(DEPDIR)/libgnu_la-argmatch.Plo
        -rm -f ./$(DEPDIR)/libgnu_la-asnprintf.Plo
        -rm -f ./$(DEPDIR)/libgnu_la-asprintf.Plo

etc.

However, the dep files corresponding to gl_LIBOBJS are not listed,
hence are not removed.

[1] https://github.com/rrthomas/recode/

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