syntax error caused by unconditional `__has_feature` usage

Encountered while working on #438.

```cpp
# if __has_feature(attribute_availability_with_strict)
# endif
```

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

See
https://en.cppreference.com/w/cpp/feature_test.html
https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005ffeature.html