> * Why iconv_t is defined as one struct (__tag_iconv_t)? [1]

Maybe it needs to be some specific type?  (At least, it doesn't need
to be void *.)

> * Where is defined the struct __tag_iconv_t? 

As the name suggests, it seems just a tag.  The internal functions
converts (casts) iconv_t objects to its own internal type (struct
_citrus_iconv * in our case), no definition is needed.

> I've been looking at GNU libiconv sources and GNU libc, and 
> iconv_t is defined as "void *" too...

I guess AbiWord inappropriately assumes GNU implementation?  If so, to
be fixed is AbiWord.

Masao