The build instructions at http://www.xfce.org/download/building refer to the package 'garcon', which does not exist in the release .tar.bz2 file for XFCE 4.8.
I now believe this to be a bug in the release, not in the web site, since the package xfce4-panel will not build without the package garcon. But I'll let you decide and reassign as appropriate.
garcon is not a core Xfce package (but an independent generic library which happens to be developed by Xfce developers). Therefore it was not included in the tarball, just as we don't include gtk, glib, etc.
OK, my mistake. Because garcon appeared in a list of XFCE packages in the build instructions, I mistakenly thought it was an XFCE package. Funnily enough, there is no Debian repository for garcon, but I have now managed to find a repository for garcon on the XFCE site. Of course I don't know that that is the best source because you have not given any clues about where the official repository is to be found.