Keep gcc -Wall happy.
diff --git a/Python/thread.c b/Python/thread.c
index 9349e58..0e6d6ee 100644
--- a/Python/thread.c
+++ b/Python/thread.c
@@ -45,6 +45,10 @@
 extern char *getenv();
 #endif
 
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+
 #include "thread.h"
 
 #ifdef __ksr__