busybox

disable format security warnings

We pass strings to printf directly a lot.  Some distros enable some flags
in their gcc by default like -Wformat-security.  With these two things, we
end up with a lot of build warnings like so:
	loginutils/chpasswd.c:42:3: warning: format not a string literal and
		no format arguments [-Wformat-security]

But we don't care.  Our focus is first and foremost on size, so adding a
lot of dummy calls like:
-	printf(some_constant_string);
+	printf("%s", some_constant_string);
is pointless bloat.

Disable this warning flag if the compiler supports it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>