-D_FORTIFY_SOUCRE has moved from CFLAGS to CPPFLAGS so you need to deal with that. I will be rebuilding it in the next day or two anyway.
None.Edit: I'm feeling helpful:https://projects.archlinux.org/archweb.git/
This is an issue caused by trying to update from an install that is almost a year old... I'm not sure my blog post will help any more.
Use a newer iso.
No - no-one has stepped up to deal with it.
Report a bug.
That probably was installed with your cross compiler too. Get rid of it.
It is very unlikely.
Hrm... I have no idea what is causing this then.