Locate the ACL2 binary via a GUI dialog or a cached path.  Attempts
to verify that the supplied executable is indeed ACL2 by looking at
the header text.

(module find-acl2 mzscheme
  (require (lib "" "framework")
           (lib "" "mred")
           (lib "")
           (lib "")
           (lib ""))
  (require "acl2-path-to-lisp-command.scm")
  (require "acl2-location-pref.scm")
  (provide find-acl2)
  (define find-acl2
    (opt-lambda ([path (get-acl2-location)])
      (if (valid-path? path)
          (let ([supplied-path
                 (finder:get-file #f "Select ACL2 Executable" (byte-regexp #".*acl2.*")
                                  "The file must contain `acl2' in the name.")])
            (and (path? supplied-path)
                 (if (valid-path? supplied-path)
                     (begin (preferences:set *acl2-loc-key* supplied-path)
                     (begin (message-box
                             "That is not ACL2"
                              "The supplied executable ~s does not appear to be ACL2."
  (define (valid-path? path)
    ;; acl2-path->command-list, in particular, may throw if user supplies
    ;; a strange `path'.
    (with-handlers ([exn:fail:read? (lambda (e) #f)])
      (let ([path (cond [(string? path) (string->path path)]
                        [(path? path) path]
                        [else #f])])
        (and (path? path)
             (file-exists? path)
             (let-values ([(proc->me me->proc pid err->me control)
                           (apply values 
                                  (apply process* 
                                         (acl2-path->command-list path)))])
               ;; Look for #"ACL2" in the output.
               (let* ([evt (regexp-match-evt #"ACL2" proc->me)]
                      [running? (eq? (control 'status) 'running)]
                      [is-acl2? (and running? (sync/timeout/enable-break 5 evt) #t)])
                 (when is-acl2? ;; ACL2 is hard to kill; you must ask it politely
                   (write-string ":q (good-bye)" me->proc)
                   (newline me->proc)
                   (flush-output me->proc))
                 (control 'kill)
                 (close-input-port proc->me)
                 (close-input-port err->me)
                 (close-output-port me->proc)