'make distcheck' reports .deps/foo.Plo files left after distclean - what to do?

[Zack Weinberg]

I just got an error from 'make distcheck' that I don't know how to resolve:

ERROR: files left in build directory after distclean:
./.deps/alg-sha512.Plo
./.deps/alg-md4.Plo
./.deps/alg-sha256.Plo
./.deps/randombytes.Plo
./.deps/alg-md5.Plo
./.deps/alg-sha1.Plo
./.deps/alg-des-tables.Plo
./.deps/alg-hmac-sha1.Plo
./.deps/alg-des.Plo

These are header-file dependency lists, autogenerated during the
build.  The source files involved are primarily used as part of a
libtool library, but their object files are also linked directly into
certain tests, because they define internal symbols that are not
exported from the fully-linked shared library (and the tests need to
exercise them directly).  This _used_ to work fine, but is apparently
now causing problems.  Possibly it is a regression in automake 1.16,
which Debian's unstable distribution just updated to.

The full Makefile.am is lengthy, you can see it at
https://github.com/besser82/libxcrypt/blob/develop/Makefile.am .
Please advise if there is anything I should be doing differently, or
if you confirm that this is a bug in automake, whether there might be
a workaround.

Thanks,
zw