Make sure 'make distclean' works for doxygen
diff --git a/Makefile.am b/Makefile.am
index e1bf86f..9086522 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -55,4 +55,7 @@
 uninstall-hook:
 	cd $(DESTDIR)$(doc_htmldir) && rm -rf {core,gsm,vty,codec}
 
+DX_CLEAN = doc/{core,gsm,vty,codec}/{html,latex}/* doc/html.tar
 endif
+
+MOSTLYCLEANFILES = $(DX_CLEAN)