modif coq-system.el to fail the coqtop-available test

coq/coq-system.el

@@ -485,6 +485,12 @@ LOADPATH, CURRENT-DIRECTORY, PRE-V85: see `coq-include-options'."
   ;; include it in the -Q options. This is not true for coqdep.
   "Build a list of options for coqc.
 LOADPATH, CURRENT-DIRECTORY, PRE-V85: see `coq-coqc-prog-args'."
+
+  ;; Modification to break the test coqtop-unavailable
+  (unless (locate-file coq-prog-name exec-path)
+    (error "coqtop is unavailable, cannot proceed"))
+
+
   (let ((topfile-supported (coq--supports-topfile)))
     (append
      ;; it is better to inform coqtop of the name of the current module