somewhat OT: a gnome build question

[Alex Snow <lists h14me homelinux net>]

Hi,
Trying to build a copy of gnome-2.20.1 from source under slackware-12.0 
to play with orca, and I run into a problem: Epiphany, Yelp, and Devhelp 
packages won't compile because they can't find gecko...the errors from 
the configure script say to install a firefox/mozilla/xulrunner dev 
package...What exactly is this looking for? I have a copy of firefox 
3.0b1 installed under /opt/firefox, and the source tree available.  How 
can I produce the development files needed by these programs from the 
mozilla source tree?