One more fix for build without gtk-doc

Fixes https://github.com/behdad/harfbuzz/pull/35

autogen.sh

diff --git a/autogen.sh b/autogen.sh
index 10a0e8a..a267f29 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -24,7 +24,7 @@
 	gtkdocize --copy || exit 1
 else
 	echo "*** No gtkdocize found, skipping documentation ***"
-	> gtk-doc.make
+	echo "EXTRA_DIST = " > gtk-doc.make
 fi
 
 echo -n "checking for autoreconf... "