Fix insecure permissions used for the top of the distfile build tree. CVE-2012-3386

--- lib/am/distdir.am.orig	Thu Jul 19 15:05:14 2012
+++ lib/am/distdir.am	Thu Jul 19 15:06:44 2012
@@ -362,7 +362,7 @@ distcheck: dist
 ## Make the new source tree read-only.  Distributions ought to work in
 ## this case.  However, make the top-level directory writable so we
 ## can make our new subdirs.
-	chmod -R a-w $(distdir); chmod a+w $(distdir)
+	chmod -R a-w $(distdir); chmod u+w $(distdir)
 	mkdir $(distdir)/_build
 	mkdir $(distdir)/_inst
 ## Undo the write access.