Fix builds outside of source tree for documentation. Potentially applicable to be sent upstream

--- libnotify-0.7.6/docs/Makefile.am 2011-08-16 04:52:04.000000000 -0700

+++ libnotify-0.7.6/docs/Makefile.am 2015-06-25 22:00:14.211074742 -0700

@@ -12,7 +12,7 @@

htmldoc_DATA = notification-spec.html

notification-spec.html: $(SPEC_XML_FILES)

- $(XMLTO) $(XMLTO_FLAGS) xhtml-nochunks -m $(top_srcdir)/docs/config.xsl notification-spec.xml

+ $(XMLTO) $(XMLTO_FLAGS) xhtml-nochunks -m $(top_srcdir)/docs/config.xsl $(top_srcdir)/docs/notification-spec.xml

endif # DOCBOOK_DOCS_ENABLED