configure: Fix build for some versions of glibc (9pfs)

Some versions declare open_by_handle_at, but don't define AT_EMPTY_PATH.
Extend the check in configure to test both preconditions.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>
