Wt Database Wt::Dbo

I am trying to learn Wt for a project and right now I am trying to learn database part of it.

I am stuck at the beginning. I am trying to learn it from dbo tutorials (there is a tutorial in the website http://www.webtoolkit.eu/wt/doc/tutorial/dbo/tutorial.html#_installing_tt_wt_dbo_tt) which is in the examples that comes within the Wt package (tutorial1.C)

I am using Qt compiler (5.) in ubuntu. I have build the Wt::Dbo library as it says in the website tutorial (linked above). the problem is it still gives an error which is;

cannot find lGL
collect2: ld returned 1 exit status

Here is the code that comes within Wt (directly copied from tutorial1.C)

/*
 * Copyright (C) 2010 Emweb bvba, Kessel-Lo, Belgium.
 *
 * See the LICENSE file for terms of use.
 */

/*****
 * This file is part of the Wt::Dbo tutorial:
 * http://www.webtoolkit.eu/wt/doc/tutorial/dbo/tutorial.html
 *****/

#include <Wt/Dbo/Dbo>
#include <Wt/Dbo/backend/Sqlite3>
#include <string>

namespace dbo = Wt::Dbo;

/*****
 * Dbo tutorial section 2. Mapping a single class
 *****/

class User {
public:
  enum Role {
    Visitor = 0,
    Admin = 1,
    Alien = 42
  };

  std::string name;
  std::string password;
  Role        role;
  int         karma;

  template<class Action>
  void persist(Action& a)
  {
    dbo::field(a, name,     "name");
    dbo::field(a, password, "password");
    dbo::field(a, role,     "role");
    dbo::field(a, karma,    "karma");
  }
};

void run()
{
  /*****
   * Dbo tutorial section 3. A first session
   *****/

  /*
   * Setup a session, would typically be done once at application startup.
   *
   * For testing, we'll be using Sqlite3's special :memory: database. You
   * can replace this with an actual filename for actual persistence.
   */
  dbo::backend::Sqlite3 sqlite3(":memory:");
  sqlite3.setProperty("show-queries", "true");
  dbo::Session session;
  session.setConnection(sqlite3);

  session.mapClass<User>("user");

  /*
   * Try to create the schema (will fail if already exists).
   */
  session.createTables();

  {
    dbo::Transaction transaction(session);

    User *user = new User();
    user->name = "Joe";
    user->password = "Secret";
    user->role = User::Visitor;
    user->karma = 13;

    dbo::ptr<User> userPtr = session.add(user);
  }

  /*****
   * Dbo tutorial section 4. Querying objects
   *****/

  {
    dbo::Transaction transaction(session);

    dbo::ptr<User> joe = session.find<User>().where("name = ?").bind("Joe");

    std::cerr << "Joe has karma: " << joe->karma << std::endl;

    dbo::ptr<User> joe2 = session.query< dbo::ptr<User> >
      ("select u from user u").where("name = ?").bind("Joe");
  }

  {
    dbo::Transaction transaction(session);

    typedef dbo::collection< dbo::ptr<User> > Users;

    Users users = session.find<User>();

    std::cerr << "We have " << users.size() << " users:" << std::endl;

    for (Users::const_iterator i = users.begin(); i != users.end(); ++i)
      std::cerr << " user " << (*i)->name
        << " with karma of " << (*i)->karma << std::endl;
  }

  /*****
   * Dbo tutorial section 5. Updating objects
   *****/

  {
    dbo::Transaction transaction(session);

    dbo::ptr<User> joe = session.find<User>().where("name = ?").bind("Joe");

    joe.modify()->karma++;
    joe.modify()->password = "public";
  }

  {
    dbo::Transaction transaction(session);
    dbo::ptr<User> joe = session.find<User>().where("name = ?").bind("Joe");
    if (joe)
      joe.remove();
  }

  {
    dbo::Transaction transaction(session);

    dbo::ptr<User> silly = session.add(new User());
    silly.modify()->name = "Silly";
    silly.remove();
  }

}

int main(int argc, char **argv)
{
  run();
}

Thanks in advance

It seems the problem occurs because of the .pro file. I am not sure how it works but the problem is gone when I replaced the pro file as I first implemented it (copy-paste from another .pro file with very little modifications and there should not be any differences between the first one the the last one).