compat: Add exit codes for Mes.

* gash/compat.scm [mes] (EXIT_SUCCESS): New variable.
[mes] (EXIT_FAILURE): New variable.

gash/compat.scm

@@ -101,7 +101,9 @@
           input-port?
           output-port?
           call-with-input-string
-          thunk?)
+          thunk?
+          EXIT_SUCCESS
+          EXIT_FAILURE)
 
   (define-macro (define-inlinable . rest)
     `(define ,@rest))
@@ -186,4 +188,7 @@
   (define (thunk? obj)
     (and (closure? obj)
          (let ((args (cadr (core:cdr obj))))
-           (or (null? args) (symbol? args))))))
+           (or (null? args) (symbol? args)))))
+
+  (define EXIT_SUCCESS 0)
+  (define EXIT_FAILURE 1))