It would probably be wise to backport this.
Also, I'm not sure how best to deal with existing packages without forcing eautoreconf all over the place.
Maybe it would be best to patch the deprecation warning out of glibc-2.25 and have it fail hard instead. Otherwise, sysmacros.h will be mis-detected by the old AC_HEADER_MAJOR macro.
On second thought, this isn't really necessary at all. Even if AC_HEADER_MAJOR mis-detects this, sys/types.h still includes sysmacros.h; it just emits a deprecation warning as well.
right, i don't think this is needed for ebuilds because we don't do -Werror. it might make sense to roll this sooner for autoconf for devs in general, and to quiet down ebuilds that happen to already run eautoreconf.
upstream wants to do a 2.70, but i'm not sure about the timeline on that. i'll keep pushing on them for it.