adding in build setup and linking external library

src/main.cpp

@@ -0,0 +1,16 @@
+#include <gtkmm.h>
+
+class MyWindow : public Gtk::Window {
+public:
+  MyWindow();
+};
+
+MyWindow::MyWindow() {
+  set_title("Basic application");
+  set_default_size(800, 450);
+}
+
+int main(int argc, char **argv) {
+  auto app = Gtk::Application::create("org.gtkmm.examples.base");
+  return app->make_window_and_run<MyWindow>(argc, argv);
+}