Joseph S. Myers writes:

sys/syscall.h uses the IS_IN_libc define before including any header that
includes libc-internal.h (where IS_IN_libc is defined).  This patch makes
sys/syscall.h include features.h to get the IS_IN_libc define where
appropriate.

include/sys/syscall.h

@@ -20,6 +20,7 @@
 #define _SYSCALL_H	1
 
 /* This file provides us with the nicely useful _syscall[0-5] macros. */
+#include <features.h>
 #if defined _LIBC && (defined IS_IN_libc || defined NOT_IN_libc)
 # include <bits/syscalls.h>
 #else