Define O_CLOEXEC for older headers that don't have it.
diff --git a/bitforce.c b/bitforce.c
index 790e000..b041aea 100644
--- a/bitforce.c
+++ b/bitforce.c
@@ -17,6 +17,9 @@
#include <termios.h>
#include <sys/stat.h>
#include <fcntl.h>
+#ifndef O_CLOEXEC
+#define O_CLOEXEC 0
+#endif
#else
#include <windows.h>
#include <io.h>