auth_socket does not build on hurd-i368

The Debian build system has for long reported, that building for architecture hurd-i386 failed. Some issues have been fixed (e.g. see MDEV-5706) but the builds still fail. Hurd is an official Debian platform,so it should be fixed.

Debian official builds overview: https://buildd.debian.org/status/package.php?p=mariadb-10.0&suite=sid

Hurd-i386 builds history: https://buildd.debian.org/status/logs.php?pkg=mariadb-10.0&arch=hurd-i386

At the moment the error is that auth_socket.so isn't built or at least that file is not found when the packages are assembled. For details see the log.