Maint: gnulib: refresh crypto/md5 module (after ~7 years, bit overdue)

Also avoid potentially arising dependency on OpenSSL -- people are
free to require that using --with-openssl, but let's keep that out
by default (as used to be the case before).

Moreover, "nutritious" dependency on kernel-provided crypto (requires
dozen of other Gnulib modoles) has explicitly been cut off as of
preceding commit.

Gnulib version used for the rebootstrap: a18f7ce3c.