Makefile: make clean clean properly

Makefile

@@ -14,6 +14,7 @@ all: Makefile.coq
 
 clean: Makefile.coq
 	+@make -f Makefile.coq clean
+	find \( -name "*.v.d" -o -name "*.vo" -o -name "*.aux" -o -name "*.cache" -o -name "*.glob" -o -name "*.vio" \) -print -delete
 	rm -f Makefile.coq
 
 Makefile.coq: _CoqProject Makefile