Detect struct in6_addr.s6_addr32 via AC_CHECK_MEMBER and use HAVE_IN6_ADDR_S6_ADDR32 instead of __linux__.
No functional changes intended, this is a preparatory refactor to keep
follow-up patches focused and easier to review.
Change-Id: I29385fde0f1e6c6cc73cd1488befdef9040cc28e
