Cross compiling: fixed compilation for older gtkmm versions

src/project.cc

@@ -174,6 +174,8 @@ Project::Clang::DebugOptionsPopover::DebugOptionsPopover() : Gtk::Popover() {
   cross_compiling_frame.add(cross_compiling_vbox);
   
   vbox.pack_start(cross_compiling_frame, true, true);
+  
+  not_yet_implemented_label.set_text("Not yet implemented");
   vbox.pack_end(not_yet_implemented_label, true, true);
   add(vbox);
   show_all();

src/project.h

@@ -66,7 +66,7 @@ namespace Project {
       Gtk::VBox cross_compiling_vbox;
       Gtk::ComboBoxText platform_list_combo_box_text;
       Gtk::Entry url_entry;
-      Gtk::Label not_yet_implemented_label=Gtk::Label("Not yet implemented");
+      Gtk::Label not_yet_implemented_label;
     };
     static std::unordered_map<std::string, DebugOptionsPopover> debug_options_popovers;