Remove legacy UnixWare support (sigwait, ipv6)

config.h.in

@@ -639,8 +639,11 @@ int sigwait(const unsigned int *set, int *sig);
 /* Define to empty if `const' does not conform to ANSI C. */
 #undef const
 
-/* Define to empty if your compiler does not support "static inline". */
+/* Define to `__inline__' or `__inline' if that's what the C compiler
+   calls it, or to nothing if 'inline' is not supported under any name.  */
+#ifndef __cplusplus
 #undef inline
+#endif
 
 /* Define to `unsigned int' if <sys/types.h> does not define. */
 #undef size_t