If not configured with --enable-cogl2-reference then don't try and dist
the doc/references/cogl-2.0 manual. Although this means a tarball built
this way will not be able to build the experimental cogl-2.0 manual this
isn't considered a big problem since anyone interested in this would
hopefully be tracking git for now or we can simply be careful to
configure with --enable-cogl2-reference for our tarball releases.
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