How to build gobject introspection ?

[Rod Butcher <rbutcher hyenainternet com>]

I've compiled gnome 2.31 with glib 3.0 for testing on my Linux x86_64
box. Everything seems to work... except gobject introspection. 
I get 
Gio-2.0.o:(.data+0x498): undefined reference to
`g_utf8_input_stream_get_type'

Can somebody  suggest where I can look to find out what's happening
here ? Also, what is gobject introspection and why is it needed ?
thanks
Rod