syscall: don't assume that WIFCONTINUED is defined

It's not defined on the Hurd.

Change-Id: Ic464f4b09b6d62bdbd34edcbf81480940eabddb7
Reviewed-on: https://go-review.googlesource.com/c/161963
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/libgo/go/syscall/wait.c b/libgo/go/syscall/wait.c
index c0c5cca..9555a41 100644
--- a/libgo/go/syscall/wait.c
+++ b/libgo/go/syscall/wait.c
@@ -16,6 +16,10 @@
 #define WCOREDUMP(status) (((status) & 0200) != 0)
 #endif
 
+#ifndef WIFCONTINUED
+#define WIFCONTINUED(x) 0
+#endif
+
 extern _Bool Exited (uint32_t *w)
   __asm__ (GOSYM_PREFIX "syscall.WaitStatus.Exited");