avoid hardlinking in case srcdir and builddir are on different filesystems

doc/graphviz/Makefile.am

@@ -12,7 +12,7 @@ link_srcdir:
 	   test ! -f $(builddir)/cache_http1_fsm.svg ; then \
 		d=`pwd`/$(builddir) ; \
 		cd $(srcdir) && find . -name \*.svg -type f | \
-		cpio -ldmp $${d} || true ; \
+		cpio -dmp $${d} || true ; \
 	fi
 
 dist-hook:

doc/sphinx/Makefile.am

@@ -32,7 +32,7 @@ clean:
 link_srcdir:
 	if test "x$(srcdir)" != "x$(builddir)" && test ! -f index.rst ; then \
 		d=`pwd`/$(builddir) ; \
-		cd $(srcdir) && find . -type f | cpio -ldmp $${d} || true ; \
+		cd $(srcdir) && find . -type f | cpio -dmp $${d} || true ; \
 	fi
 
 # work around for make html called within doc/sphinx