Test for CFLAGS/LFLAGS availability

Use the CC_CHECK_FLAGS_APPEND() m4 macro to test for availability of
CFLAGS and LFLAGS. It automatically drops the unavailable flags.

This is also used by systemd, so it ought to work with ostree and other
non-standard build-environments.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reintroduce -fstack-protector

We removed this previously due to build failures in:

  commit 14ac764ec86452ca607403f314b0f8355d80290c
  Author: David Herrmann <dh.herrmann@gmail.com>
  Date:   Mon Sep 9 16:03:41 2013 +0200

      build: remove -fstack-protector

Reintroduce it but this time disable it if it's not supported. We use the
CC-flags testing to prevent build-failures on gcc without libssp support.

Cc: Giovanni Campagna <gcampagn@redhat.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>