XBMC recently added support for using external libraries inside of bundling every single dependency. See http://xbmc.org/trac/ticket/5416 for details.
Basically, "--enable-external-libraries" needs to be added to configure, and the ebuild needs to be changed to depend on the libraries it needs.
updated in cvs, thanks
i recommend this bug be reopened, because enabling external libraries breaks xbmc python . there might be a way to somehow configure xbmc to work better with system python, but i'd rather have saved all this time it took figuring this out. i tried --enable-external-libraries and --disable-external-python, but --e-e-l always prevailed. xbmc svn 24501 compiled fine with all internals as well, so at least a static use flag should be provided for this.
I recommend this bug be reopened too, especially a useflag for external ffmpeg would be nice (ffmpeg maybe), because xbmc-9999 is at the moment only compatible to ffmpeg and not libav. To circumvent this problem it would be nice to disable the ffmpeg useflag (which internally set --disable-external-ffmpeg), so xbmc uses its own ffmpeg and not the systemwide. See also https://bugs.gentoo.org/show_bug.cgi?id=420831.