recently added to stddef.h behind a conditional. There is a gnulib problem
when detecting this (here it's because tests run with --std=gnu99 but some
code is built with --std=gnu++11, exposing the definition in the system
headers, and causing a conflict). Figured out by kettenis@.
This is the only port affected so it makes sense to fix it here rather than
provide a workaround in the system headers. ok jasper@ (maintainer).
4f7bc9590b