build: improve the clean target

* Uses a sentinel file for the obj directory.
* Removes the binary with 'make clean'.
* Uses 'rm -f` to ensure the clean target always succeeds.

Makefile

@@ -83,14 +83,18 @@ ifeq ($(TARGET),gtk)
 endif
 
 all: $(FNAME_$(TARGET))
-obj:
-	mkdir obj
-clean: | obj
-	rm obj/* || true
+
+obj/.tag:
+	@mkdir -p obj
+	@touch $@
+
+clean:
+	rm -rf obj/
+	rm -f $(FNAME_$(TARGET))
 
 ifeq ($(TARGET),windows)
   XFILES += obj/rc.o
-obj/rc.o: flips.rc flips.h | obj
+obj/rc.o: flips.rc flips.h obj/.tag
 	windres flips.rc obj/rc.o
 endif