Another try to make gtk-doc optional
diff --git a/docs/reference/Makefile.am b/docs/reference/Makefile.am
index f7a4ad6..4bdc48e 100644
--- a/docs/reference/Makefile.am
+++ b/docs/reference/Makefile.am
@@ -88,7 +88,7 @@
 endif
 
 # This includes the standard gtk-doc make rules, copied by gtkdocize.
-include $(top_srcdir)/gtk-doc.make
+-include $(top_srcdir)/gtk-doc.make
 
 # Other files to distribute
 # e.g. EXTRA_DIST += version.xml.in