syntax error caused by unconditional `__has_include_next` usage

Encountered while working on #438.

```cpp
# if __has_include_next(<cstddef>)
# endif
```

```
a.cpp:1: syntax error: failed to evaluate #if condition, undefined function-like macro invocation: __has_include_next( ... )
```

See https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005finclude.html.