mescc: Exit with non-zero exit code when subprocess fails.

system* returns the result of waitpid. So, suppose that a subprocess
fails with exit code 1. Then the waitpid return value will be 256.
And exit(256) is equivalent to exit(0).

Modified-by: Jan Nieuwenhuizen <janneke@gnu.org>

* mes/module/mes/posix.mes (status:exit-val): New function.
* module/mescc/mescc.scm (assert-system*): Use it.

mes/module/mes/posix.mes

@@ -57,3 +57,6 @@
 (define (waitpid pid . options)
   (let ((options (if (null? options) 0 (car options))))
     (core:waitpid pid options)))
+
+(define (status:exit-val status)
+  (ash status -8))

module/mescc/mescc.scm

@@ -281,7 +281,7 @@
   (let ((status (apply system* args)))
     (when (not (zero? status))
       (stderr "mescc: failed: ~a\n" (string-join args))
-      (exit status))
+      (exit (status:exit-val status)))
     status))
 
 (define (multi-opt option-name) (lambda (o) (and (eq? (car o) option-name) (cdr o))))