Add aseigo's composite extension testing code, as per comment at <a href="http://dot.kde.org/1179921215/1179939880/1179940099/">http://dot.kde.org/1179921215/1179939880/1179940099/</a>;

It builds, installs, runs, does not appear to break for kwin composite enabled or disabled, so ship it!

(there is the old line, still commented out that appears to now be redundant - should be removed if there are no problems with the new one...)