syntax error caused by unconditional `__has_cpp_attribute` usage

Encountered while working on #438.

```cpp
# if __has_cpp_attribute(maybe_unused)
# endif
```

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

See
https://en.cppreference.com/w/cpp/feature_test.html#Attributes
https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005fcpp_005fattribute.html