syntax error caused by unconditional `__has_extension` usage

Encountered while working on #438.

```cpp
# if __has_extension(cxx_rvalue_references)
# endif
```

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

See
https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005fextension.html