Don't attempt to create ChangeLogs if not in a git-tree.

There is no need to copy the files in that case thanks to VPATH.
That assumes you are using a source tarball then - if not, you are
screwed anywhow.
(cherry picked from commit 16402c26)

doc/Makefile.am

@@ -50,11 +50,16 @@ 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="$$y-12-31 23:00:00 -0100" \
-		> "$@"
+	if test -d "$(top_srcdir)/.git"; then \
+		y="$$(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
+#
+#distclean-hook:
+#	test "$(srcdir)" = "$(builddir)" || rm -f -- $(CHANGELOGS)