Alas illumos doesn't define s6_addr8, s6_addr16, and s6_addr32 outside the kernel.
But netinet/in.h gives an idea how to provide the definitions.

--- swipl-9.2.9/packages/clib/nonblockio.c.orig
+++ swipl-9.2.9/packages/clib/nonblockio.c
@@ -182,6 +182,8 @@
 #define s6_addr16 s6_words
 #elif defined(__APPLE__) || defined(__FreeBSD__) || defined(__OpenBSD__)
 #define s6_addr16 __u6_addr.__u6_addr16
+#elif defined(__illumos__)
+#define s6_addr16   _S6_un._S6_u16
 #endif
 #endif