update Makefile

Makefile

@@ -9,7 +9,7 @@ all: Makefile.coq
 clean: Makefile.coq
 	+@make -f Makefile.coq clean
 	find theories tests \( -name "*.d" -o -name "*.vo" -o -name "*.aux" -o -name "*.cache" -o -name "*.glob" -o -name "*.vio" \) -print -delete || true
-	rm -f Makefile.coq
+	rm -f Makefile.coq .lia.cache
 .PHONY: clean
 
 # Create Coq Makefile.