Undefiend reference to vtable 'ubidots_init(char*)'

Hello,

I’ve started using Ubidots with C language but i had a problem i couldn’t solve.
I’ve downloaded the Source files and included them into my ‘include’ folder ( I use both Codeblock and Qt ), but i’ve this problem popping after compiling my project .

undefined reference to `ubidots_init(char*)'

My code is simple : ( just a basic incompleted hello world ^^’ )

#include <iostream>
#include <Ubidots/ubidots.h>  // The hierarhy is: include/Ubidots/ubidots.h

using namespace std;

int main()
{
    UbidotsClient* client = ubidots_init( "some_api_key" );
    cout << "Hello world!" << endl;
    return 0;
}

Basicaly, it happens with every single function from ubidots.h ( for exemple: ubidots_cleanup too )
Thanks for helping me.

Cordialy.

hello !
how did you solve the problem ?