ubuntu16.04 c++11 std::mutex invalid

suppose there're two threads execute the function `f` at the same time, like this:

int g_i = 0;
std::mutex g_mtx; 

void f() 
{
  for (int i = 0; i < 10000000; ++i) {
    std::lock_guard<std::mutex> lck(g_mtx);
    g_i++;
  }
}

the final result is not 20000000 instead of a random integer less than 20000000, even though I has append the options :

target_link_libraries(test pthread)

in the CMakeLists.txt.Obviously the mutex is invalid.

Buf if i compile the test cpp file by command line with -pthread options, the result is correct.

So how to fix this problem?