Declare variable in namespace, define it in main, make it visible to all other files

Using C++14, I'm trying to define a variable in a namespace where commonly used variables are stored (App::Common). The main function should be the one that sets it, since it gets set to argv[0]. Meanwhile I need the variable to be visible by all other classes/files. But I get the linker error shown below. Also, ideally I would like the variable to be const where only the main function would set it once.

common.hpp

#pragma once
#include <string>
namespace App{
    namespace Common{
        extern std::string appPath;
    }
}

main.cpp

#include "common.hpp"
#include "client.hpp"
#include <string>
int main() {
    App::Common::appPath = argv[0];
}

client.hpp

#include "common.hpp"
class Client {
    public:
    void printAppPath();
};

client.cpp

#include <iostream>
#include <string>
#include "common.hpp"
#include "client.hpp"
void Client::printAppPath() {
    std::cout << App::Common::appPath << std::endl;
}

I get the following error by the linker:

ld: main.o: in function `main':
main.cpp:(.text.startup.main+0x25): undefined reference to `App::Common::appPath[abi:cxx11]'
ld: Client.o: in function `Client::printAppPath()':
Client.cpp:(.text...): undefined reference to `App::Common::appPath[abi:cxx11]'