mruby-exit/exit.c: add fallback definition of constants.

- EXIT_SUCCESS
- EXIT_FAILURE

mrbgems/mruby-exit/src/mruby-exit.c

@@ -1,6 +1,14 @@
 #include <stdlib.h>
 #include <mruby.h>
 
+#ifndef EXIT_SUCCESS
+# define EXIT_SUCCESS 0
+#endif
+
+#ifndef EXIT_FAILURE
+# define EXIT_FAILURE 1
+#endif
+
 static mrb_value
 f_exit(mrb_state *mrb, mrb_value self)
 {