fix calling individual targets

(broken in 82af922b40)

Otherwise commands like `make i2p` will fail.

Makefile

@@ -37,10 +37,12 @@ api: $(SHLIB)
 ## custom FLAGS to work at build-time.
 
 deps:
+	@test -d obj || mkdir obj
 	$(CXX) $(CXXFLAGS) $(NEEDED_CXXFLAGS) -MM *.cpp > $(DEPS)
 	@sed -i -e '/\.o:/ s/^/obj\//' $(DEPS)
 
 obj/%.o : %.cpp
+	@test -d obj || mkdir obj
 	$(CXX) $(CXXFLAGS) $(NEEDED_CXXFLAGS) $(INCFLAGS) $(CPU_FLAGS) -c -o $@ $<
 
 # '-' is 'ignore if missing' on first run