HAS_POST_2038 test is environment-dependent

On my machine the following results are returned by strptime:

```
time2037.tm_hour = 13; // weird
time2038.tm_hour = 12; // correct
```

This results in time difference 31532400 (which is 3600 less than expected). Some other configurations give the expected result, thus making the build variable to flap during sequential builds on different systems.