autoconf: add patch to make it work under dash

autoconf >= 2.64 was broken when /bin/sh is symlinked to dash. This
has been fixed post-2.65 by the commit at
http://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=7f75858f577f11a844781764f30cd42cfe8a5669

.

Compared to the original patch, the patch included in Buildroot
manually does the change to the ./configure script itself. Otherwise,
the compilation of host-autoconf would require host-autoconf itself.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>