Allow make doxygen to work properly

git-svn-id: http://svn.fedorahosted.org/svn/corosync/trunk@1418 fd59a12c-fef9-0310-b244-a6a79926bd2f

Makefile

@@ -198,4 +198,4 @@ endif
 	install -m 644 man/*.8 $(DESTDIR)$(MANDIR)/man8
 
 doxygen:
-	doxygen
+	mkdir -p doc/api && doxygen