syntax error caused by unconditional `__has_builtin` usage

Encountered while working on #438.

```cpp
#if __has_builtin(__builtin_bit_cast)
#endif
```

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

See https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005fbuiltin.html.