fixes XE6 build.

This commit was SVN r25600.

orte/mca/plm/rsh/plm_rsh_module.c

@@ -572,9 +572,9 @@ static int setup_launch(int *argcptr, char ***argvptr,
     }
 
     value = opal_argv_join(argv, ' ');
-    if (ARG_MAX < strlen(value)) {
+    if (sysconf(_SC_ARG_MAX) < strlen(value)) {
         orte_show_help("help-plm-rsh.txt", "cmd-line-too-long",
-                       true, strlen(value), ARG_MAX);
+                       true, strlen(value), sysconf(_SC_ARG_MAX));
         free(value);
         return ORTE_ERR_SILENT;
     }