build: fix bootstrap with older autoconf

Pavel Hrdina reported to me off-list that my gnulib update on
Jan 1 broke the build on RHEL 6.4 (autoconf 2.63) and older:

    executing aclocal -I glm4
    glm4/gl-openssl.m4:11: error: m4_defn: undefined macro: _m4_divert_diversion
    glm4/gl-openssl.m4:11: the top level
    autom4te: /usr/bin/m4 failed with exit status: 1
    aclocal: autom4te failed with exit status: 1

It took me a while, but I fixed the regression in gnulib.

* gnulib: Update to latest, for build fixes.
Signed-off-by: NEric Blake <eblake@redhat.com>

.gnulib @ d18d1b80

-Subproject commit c7802e2b418c40fa2bd270cf241314d0c8433a26
+Subproject commit d18d1b8023822220bb8f0a079c7312a1adffdce0