Accept `__has_include` for GNU C standards.

Using `__has_include` with `-std=gnu...` currently fails for C standards and C++ standards < 17:

`file.c`:
```c
#if __has_include(<dir/file.h>)
#endif
```

Output:
```console
$ ./simplecpp -std=gnu99 -e file.c
file.c:1: syntax error: failed to evaluate #if condition, division/modulo by zero
```
