Three recent changes use what only Linux has, so libc and two tests no
longer compile on macOS:
- The AF_LINK entries that getifaddrs lists on Linux use linux/if_link.h,
netpacket/packet.h, sockaddr_ll and rtnl_link_stats, and were built on
every host but Windows. They are now built on Linux only: macOS lists
the IPv4 and IPv6 addresses as before, as Windows does, and the
interface test checks the AF_LINK entries on Linux only.
- The NetCtlAp test loops over a braced list without including
<initializer_list>, which libc++ does not pull in through the test's
other headers.
- The import block test backs its 16 KiB views with memfd_create. The
views case is now built on Linux only; macOS runs the case without
views, as Windows does.
