Follow-up to 299b01d

Add select_close GUIType and process it
+ Field::Choice: Fix a check of m_opt.enum_def existence
+ Fixed OSX build

src/libslic3r/PrintConfig.cpp

@@ -334,7 +334,7 @@ void PrintConfigDef::init_common_params()
     def = this->add("printhost_port", coString);
     def->label = L("Printer");
     def->tooltip = L("Name of the printer");
-//    def->gui_type = ConfigOptionDef::GUIType::select_open;
+    def->gui_type = ConfigOptionDef::GUIType::select_close;
     def->mode = comAdvanced;
     def->cli = ConfigOptionDef::nocli;
     def->set_default_value(new ConfigOptionString(""));