Building busybox will fail with errors in network/tc.c because CBQ has been removed from the linux kernel. Upstream is already aware of this : https://bugs.busybox.net/show_bug.cgi?id=15931. I know upstream bugs shouldn't be reported, but a temporary workaround would be to use one of the patches on the page disabling CBQ: https://bugs.busybox.net/attachment.cgi?id=9760.
This should be fixed already in bug 926872. *** This bug has been marked as a duplicate of bug 926872 ***
I suspect you're (unnecessarily) using ~arch linux-headers on stable. Anyway, filed bug 938267.