scripts: exporting (even empty) LIBRARY_PATH and/or LPATH breaks gcc

If either LIBRARY_PATH or LPATH is set, even to the empty string,
the gcc build breaks.

Fix that by bailing-out rather than re-setting.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

#!/bin/sh

set -e

printf "Running autoconf...\n"

autoconf -Wall --force

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