With commit 3b6b078e4d it's now possible to build XBMC with optional UPnP support. Please add a upnp use flag.
should be all set now in the tree; thanks for the report! Commit message: Add USE=upnp support http://sources.gentoo.org/media-tv/xbmc/xbmc-9999.ebuild?r1=1.116&r2=1.117