scripts: remove duplicate '/' in prefix, it breaks relocation

Reported-by: Richard Weinberger <richard.weinberger@gmail.com>
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"