Support "named" variadic macros

This piece of code appears in recent Ubuntu distributions, in `/usr/include/linux/stddef.h`:

```c++
#define __struct_group(TAG, NAME, ATTRS, MEMBERS...) \
union { \
struct { MEMBERS } ATTRS; \
struct TAG { MEMBERS } ATTRS NAME; \
}
```

The [gcc manual](https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html) notes that names other than `__VA_ARGS__` - in this case, `MEMBERS` - may be used to refer to the variadic arguments "as an extension". Given the ubiquity of this header it may be useful to add support for this extension.