Some pre-POSIX systems don't have unistd.h (but e.g. lib.c).
Allow configuring the name of that header file.
diff --git a/crypto/idea/idea_spd.c b/crypto/idea/idea_spd.c
index 3c24ff5..5f79f93 100644
--- a/crypto/idea/idea_spd.c
+++ b/crypto/idea/idea_spd.c
@@ -64,12 +64,11 @@
#endif
#include <stdio.h>
-#ifndef MSDOS
-#include <unistd.h>
-#else
-#include <io.h>
-extern int exit();
-#endif
+
+#include <openssl/e_os2.h>
+#include OPENSSL_UNISTD_IO
+OPENSSL_DECLARE_EXIT
+
#include <signal.h>
#ifndef _IRIX
#include <time.h>
