https://github.com/plougher/squashfs-tools/commit/fa862f08f15f1beadfeb39122cb976057160bf14

Index: squashfs-tools/signals.h
--- squashfs-tools/signals.h.orig
+++ squashfs-tools/signals.h
@@ -28,7 +28,7 @@ static inline int wait_for_signal(sigset_t *sigmask, i
 {
 	int sig;
 
-#if defined(__APPLE__) && defined(__MACH__)
+#if (defined(__APPLE__) && defined(__MACH__)) || defined(__OpenBSD__)
 	sigwait(sigmask, &sig);
 	*waiting = 0;
 #else
