Created attachment 295579 [details] [review]
gtype: allow forward declarations for circular types

This allows multiple declarations such as:

 typedef struct _Foo Foo;

as is needed when you have systems with circular referencing.

Pushed with a slightly tweaked commit message summary line.

Thanks!

fwiw, I was going to do the same for the Class struct definition in the
_FINAL_ variant of this macro until I realised that it would make no
sense at all.  That struct is defined only for convenience and it really
ought not to be declared anywhere else at all if the class is final.