How do I use bcrypt in c++ under Linux using wrapper libbcrypt

I try to write a simple login script using bcrypt. I've tried to use the C wrapper libbcrypt library - https://github.com/trusch/libbcrypt - but got stucked with the compiler complaining about undefinied references to bcrypt_checkpw, bcrypt_hashpw, bcrypt_gensalt in the BCrypt class.

First thing I've done was to follow the instructions from the readme.md - https://github.com/trusch/libbcrypt/blob/master/README.md :

git clone https://github.com/trusch/libbcrypt
cd libbcrypt
mkdir build
cd build
cmake ..
make
sudo make install
sudo ldconfig

Then I have my main.cpp file

#include <iostream>
#include "User_Class.hpp"

void checkPermission();

int main(int argc, char* argv[]) {    
    checkPermission();
    return 0;
}

void checkPermission() {
    User_Class User;
    User.Login("test");
}

My User_Class.cpp file

#include <iostream>
#include "Password_Class.hpp"
#include "User_Class.hpp"

bool User_Class::Login(std::string password) {
    return Password_Class::authenticate(password);
}

My User_Class.hpp file

class User_Class {
    public:
        bool Login(std::string password);
};

My Password_Class.cpp file

#include <iostream>
#include "Password_Class.hpp"
#include "bcrypt/BCrypt.hpp"

bool Password_Class::authenticate(std::string password) {

    // this fails!
    std::string hash = BCrypt::generateHash(password);  
    std::cout << BCrypt::validatePassword(password,hash) << std::endl;

    return true;

}

And my Password_Class.hpp file

class Password_Class {
    public:
        static bool authenticate(Glib::ustring password);
};

When compiling with this command

g++ -lbcrypt main.cpp User_Class.cpp Password_Class.cpp -o main `pkg-config --libs --cflags gtkmm-3.0`

I get errors

/tmp/ccc3Va1h.o: In Funktion »BCrypt::generateHash(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int)«:
Password_Class.cpp:(.text._ZN6BCrypt12generateHashERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEi[_ZN6BCrypt12generateHashERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEi]+0x44): Warnung: undefinierter Verweis auf »bcrypt_gensalt«
Password_Class.cpp:(.text._ZN6BCrypt12generateHashERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEi[_ZN6BCrypt12generateHashERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEi]+0xb0): Warnung: undefinierter Verweis auf »bcrypt_hashpw«
/tmp/ccc3Va1h.o: In Funktion »BCrypt::validatePassword(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)«:
Password_Class.cpp:(.text._ZN6BCrypt16validatePasswordERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEES7_[_ZN6BCrypt16validatePasswordERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEES7_]+0x33): Warnung: undefinierter Verweis auf »bcrypt_checkpw«
collect2: error: ld returned 1 exit status

So please can someone tell me what's going on here? What have I done wrong so how do I use bcrypt in C++? I guess some libararies couldn't be found but I'm really no expert in linux or C++ so please forgive me ;)

Thank you for your help!