Very old systems (pre XPG 4.2) may not support MSG_WAITALL. The

likelihood of receiving a partial message is quite low so this is
not a big deal.

src/sudo_exec.h

@@ -17,6 +17,13 @@
 #ifndef _SUDO_EXEC_H
 #define _SUDO_EXEC_H
 
+/*
+ * Older systems may not support MSG_WAITALL but it shouldn't really be needed.
+ */
+#ifndef MSG_WAITALL
+# define MSG_WAITALL 0
+#endif
+
 /*
  * Special values to indicate whether continuing in foreground or background.
  */