cc: add a flag for skipping core passes

It is used for skipping unnecessary compilation steps when the libc
doesn't need to be compiled (eg. when we do not use a C library).

Signed-off-by: Yann Diorcet <diorcet.yann@gmail.com>
Message-Id: <150eadb0117e697d79aa.1353625025@blackmint>
Patchwork-Id: 201222

#!/bin/sh

set -e

printf "Running autoconf...\n"

autoconf -Wall --force

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