This incorporates:
2018-01-01 maint: Run 'make update-copyright'
2017-12-29 Add cross-compilation results for GNU/Hurd.
2017-12-12 explicit_bzero: port to macOS + Clang 9.0.0
* admin/merge-gnulib (GNULIB_MODULES): Add explicit_bzero.
* lib/explicit_bzero.c, m4/explicit_bzero.m4: New files.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* src/gnutls.c (clear_storage): New function.
(gnutls_symmetric_aead): Use it instead of memset.