fix `raco setup -j' for a non-places build

When places are not enabled, then `raco setup' uses multiple
Racket processes that commuincate via pipes. Don't try to send
a place channel over a pipe.

Merge to v5.3.4
(cherry picked from commit 2295d16074)

collects/setup/scribble.rkt

@@ -69,11 +69,12 @@
   (setup-printf "error running" (module-path-prefix->string (doc-src-spec doc)))
   (eprintf errstr))
 
-;; We use a lock to control writing to the database, because
-;; the database or binding doesn't seem to deal well with concurrent
-;; writers within a process.
+;; We use a lock to control writing to the database. It's not
+;; strictly necessary, but place channels can deal with blocking
+;; more efficiently than the database connection.
 (define no-lock void)
 (define (lock-via-channel lock-ch)
+  (if lock-ch
       (let ([saved-ch #f])
         (lambda (mode)
           (case mode
@@ -83,11 +84,16 @@
              (set! saved-ch ch)]
             [(unlock)
              (place-channel-put saved-ch 'done)
-         (set! saved-ch #f)]))))
+             (set! saved-ch #f)])))
+      void))
 (define lock-ch #f)
 (define lock-ch-in #f)
 (define (init-lock-ch!)
   (unless lock-ch
+    ;; If places are not available, then tasks will be run
+    ;; in separate OS processes, and we can do without an
+    ;; extra lock.
+    (when (place-enabled?)
       (set!-values (lock-ch lock-ch-in) (place-channel))
       (thread (lambda ()
                 (define-values (ch ch-in) (place-channel))
@@ -95,7 +101,7 @@
                   (place-channel-put lock-ch-in ch)
                   (place-channel-get ch-in)
                   (place-channel-get ch-in)
-                (loop))))))
+                  (loop)))))))
 (define (call-with-lock lock thunk)
   (lock 'lock)
   (dynamic-wind