Avoid race condition when using multiple make jobs

Avoid race condition when symlinking XML files.
When declaring a rule with many files as target, the rule is called
when any of them is requested, resulting in multiple for loops happening
during a make process using more than one job.
Also, use '$(LN_S) -f' rather than removing and recreating a file,
that one should be as supported as 'rm -f' and requires one less command.

src/Makefile.am

@@ -220,7 +220,4 @@ SUFFIXES = .xml
 	            -o $@ $(srcdir)/c-client.xsl $< 
 
 $(EXTENSION_XML):
-	for i in $(EXTENSION_XML) ; do \
-           rm -f $$i ; \
-           $(LN_S) $(XCBPROTO_XCBINCLUDEDIR)/$$i $$i ; \
-        done
+	$(LN_S) -f $(XCBPROTO_XCBINCLUDEDIR)/$@ $@