Summary:
Issue google#80 points out several places in glog where TSAN discovers
false positives. One of these places is in the `LOG_EVERY_N` macros.
These macros are implemented by maintaining a static unprotected
integer counter, and TSAN will report data races on these counters.

Here is a minimum example to reproduce the data race:

```

void logging() {
  for (int i = 0; i < 300; ++i) {
    LOG_EVERY_N(INFO, 2) << "foo";
  }
}

int main() {
  auto t1 = std::thread(logging);
  auto t2 = std::thread(logging);
  t1.join();
  t2.join();
  return 0;
}
```

And here is the TSAN report:

```
WARNING: ThreadSanitizer: data race (pid=776850)
  Write of size 4 at 0x558de483f684 by thread T2:
    #0 logging()
    google#1 void std::_Bind_simple<void (*())()>::_M_invoke<>(std::_Index_tuple<>)
    google#2 std::_Bind_simple<void (*())()>::operator()()
    google#3 std::thread::_Impl<std::_Bind_simple<void (*())()> >::_M_run()
    google#4 execute_native_thread_routine

  Previous write of size 4 at 0x558de483f684 by thread T1:
    #0 logging()
    google#1 void std::_Bind_simple<void (*())()>::_M_invoke<>(std::_Index_tuple<>)
    google#2 std::_Bind_simple<void (*())()>::operator()()
    google#3 std::thread::_Impl<std::_Bind_simple<void (*())()> >::_M_run()
    google#4 execute_native_thread_routine

  Location is global '<null>' at 0x000000000000 (main+0x00000011c684)

  Thread T2 (tid=776857, running) created by main thread at:
    #0 pthread_create
    google#1 __gthread_create
    google#2 std::thread::_M_start_thread(std::shared_ptr<std::thread::_Impl_base>, void (*)())
    google#3 main

  Thread T1 (tid=776856, running) created by main thread at:
    #0 pthread_create
    google#1 __gthread_create
    google#2 std::thread::_M_start_thread(std::shared_ptr<std::thread::_Impl_base>, void (*)())
    google#3 main

SUMMARY: ThreadSanitizer: data race in logging()
```

To avoid noisy TSAN reports and also avoid adding a performance hit, this
change will mark these counters as benign races so that TSAN will not report
them. This change will only have an effect if we are compiling with TSAN;
there are no changes if we are not building with TSAN.

With this change, the above example no longer reports a data race when built
and run with TSAN.