gnulib: Pull in latest changes

In particular, we're interested in the following commits:

  commit 43b5194d5b156f8dd7ae576952568d331978f5f0
  Author: Bruno Haible <bruno@clisp.org>
  Date:   Sun Jan 5 20:42:12 2020 +0100

    tests: Avoid GCC over-optimization caused by _GL_ARG_NONNULL attributes.

    * lib/stdlib.in.h: Tweak last commit.

  commit b7d7afe10ddf599452bd80b8a840c830cd474b09
  Author: Bruno Haible <bruno@clisp.org>
  Date:   Sun Jan 5 09:13:25 2020 +0100

    tests: Avoid GCC over-optimization caused by _GL_ARG_NONNULL attributes.

    Reported by Jim Meyering in
    <https://lists.gnu.org/archive/html/bug-gnulib/2020-01/msg00040.html>.

    * lib/stdlib.in.h (GNULIB_defined_canonicalize_file_name): New macro.
    (GNULIB_defined_ptsname_r): New macro.
    * tests/test-canonicalize.c (_GL_ARG_NONNULL): Define to empty.
    (main): Disable the NULL argument test if canonicalize_file_name does
    not come from gnulib.
    * tests/test-canonicalize-lgpl.c (_GL_ARG_NONNULL): Define to empty.
    (main): Disable the NULL argument test if canonicalize_file_name does
    not come from gnulib.
    * tests/test-ptsname_r.c (_GL_ARG_NONNULL): Define to empty.
    (test_errors): Disable the NULL argument test if ptsname_r does not come
    from gnulib.

since they fix a build failure caused by the gnulib tests failing
on ppc64le, as reported in

  https://www.redhat.com/archives/libvir-list/2020-January/msg00616.htmlReported-by: NSatheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Tracked-down-by: NBruno Haible <bruno@clisp.org>
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>

.gnulib @ 611869be

-Subproject commit 7d069378921bfa0d7c7198ea177aac0a2440016f
+Subproject commit 611869be9f1083e53305446d90a2909fc89914ef