Only do "extern int errno" if errno is not a macro.

sudo.h

@@ -248,6 +248,8 @@ extern uid_t timestamp_uid;
 
 extern void (*set_perms) __P((int));
 #endif
+#ifndef errno
 extern int errno;
+#endif
 
 #endif /* _SUDO_SUDO_H */

sudo_noexec.c

@@ -48,7 +48,9 @@ static const char rcsid[] = "$Sudo$";
  * few programs actually do that.
  */
 
+#ifndef errno
 extern int errno;
+#endif
 
 #define DUMMY(fn, args, atypes)	\
 int				\