docs/known-issues: add note about build issue on x86_64 with 32bit userspace

Signed-off-by: Johannes Stezenbach <js@sig21.net>
(transplanted from da77b5e7cb9f921d0e2a22818b7945906100e704)

 #!/bin/sh

 set -e

 printf "Running autoconf...\n"

 autoconf -Wall --force

 printf "Done. You may now run:\n    ./configure\n"