build: Explicitly flush the "LOAD" and "GUILEC" lines.

* build-aux/compile-all.scm <top level>: Add calls to 'force-output'
in the #:report-load and #:report-compilation procedures.  Fixes a
regression introduced in 2890ad332f
whereby compilation output would be buffered, leading to a weird
visual effect.

build-aux/compile-all.scm

@@ -92,8 +92,10 @@ to 'make'."
                   #:host host
                   #:report-load (lambda (file total completed)
                                   (when file
-                                    (format #t "  LOAD     ~a~%" file)))
+                                    (format #t "  LOAD     ~a~%" file)
+                                    (force-output)))
                   #:report-compilation (lambda (file total completed)
                                          (when file
                                            (format #t "  GUILEC   ~a~%"
-                                                   (scm->go file)))))))
+                                                   (scm->go file))
+                                           (force-output))))))