Copy the changelogs if they exist instead of generating them with git

Fix make distcheck
(cherry picked from commit 70f620e7f94eb02fc6f5eda47599ae5354c12483)

doc/Makefile.am

@@ -50,11 +50,15 @@ EXTRA_DIST = \
 DISTCLEANFILES = $(CHANGELOGS)
 
 $(CHANGELOGS): Makefile.am
-	y="$$(echo "$@" | sed -e 's,ChangeLog-,,')" ; \
-	git --git-dir=$(top_srcdir)/.git log\
-		--since="$$y-01-01" \
-		--until="$$y-12-31 23:00:00 -0100" \
-		> "$@"
+	if test -f "$(top_srcdir)/doc/$@"; then \
+		cp "$(top_srcdir)/doc/$@" "$@" ; \
+	else \
+		y="$$(echo "$@" | sed -e 's,ChangeLog-,,')" ; \
+		git --git-dir=$(top_srcdir)/.git log \
+			--since="$$y-01-01" \
+			--until="$$y-12-31 23:00:00 -0100" \
+			> "$@" ; \
+	fi
 
 # This one needs to be rebuilt all the time :)
 .PHONY: ChangeLog-2008