error: constexpr variable 'is_digit' must be initialized by a constant expression

```
C:/msys64/home/Administrator/ObEngine/extlibs/vili/src/utils.cpp:11:20: error: constexpr variable 'is_digit' must be initialized by a constant expression
    constexpr auto is_digit = static_cast<int (*)(int)>(std::isdigit);
                   ^          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```

Change `constexpr` to `const` solved the problem.