In order to use the strigi engine in KBFX you have to do these steps:
Copy the "strigi-X.X.X.ebuild" and the "kbfx-0.4.9.X.ebuild" (from the sources directory) to your local portage tree as needed OR use the "kbfx-0.4.9.X-portage-overlay.tar.bz2"
Set the flag "strigi" for the kbfx ebuild
There is a Layman overlay resource on the SourceForge SVN. You can add it by this command:
> layman -f -o https://kbfx.svn.sourceforge.net/svnroot/kbfx/layman/kbfx-overlay.xml \ -a kbfx
After you add the "kbfx" overlay you can use it as usual.