List xproto.xml path explicitly instead of relying on GNU Make $< expansion

(Some non-gnu makes, such as Solaris make, only recognize $< in implicit
 suffix rules, not explicit ones like this.)

src/Makefile.am

@@ -221,7 +221,7 @@ SUFFIXES = .xml
 	            -o $@ $(srcdir)/c-client.xsl $< 
 
 xproto.xml: $(XCBPROTO_XCBINCLUDEDIR)/xproto.xml
-	$(LN_S) $< $@
+	$(LN_S) $(XCBPROTO_XCBINCLUDEDIR)/xproto.xml $@
 
 $(EXTENSION_XML):
 	[ -d extensions ] || mkdir extensions