Improved support for IBM operating systems

Fix compiler errors and warnings when building on IBM operating systems such as AIX and IBM i.

strerror_override.c

@@ -20,7 +20,9 @@ static struct
 	ENTRY(EIO),
 	ENTRY(ENXIO),
 	ENTRY(E2BIG),
+#ifdef ENOEXEC
 	ENTRY(ENOEXEC),
+#endif
 	ENTRY(EBADF),
 	ENTRY(ECHILD),
 	ENTRY(EDEADLK),