XXX not portable, but used by tests only.

Index: tests/common/thread-id.h
--- tests/common/thread-id.h.orig
+++ tests/common/thread-id.h
@@ -69,6 +69,14 @@ unsigned long urcu_get_thread_id(void)
 	return pthread_getsequence_np(&thr);
 }
 
+#elif defined(__OpenBSD__)
+# include <unistd.h>
+
+static inline
+unsigned long urcu_get_thread_id(void)
+{
+	return (unsigned long) getthrid();
+}
 #else
 # warning "use pid as thread ID"
 static inline