docs: don't dist cogl-2.0 manual if not built
docs: don't dist cogl-2.0 manual if not built

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.

heysion/clutter-clone,GNOME/clutter,jigpu/clutter,djdeath/clutter,heysion/clutter-clone,kerrickstaley/clutter-vala,dlespiau/clutter,collects/clutter,jigpu/clutter,Distrotech/clutter,djdeath/clutter,collects/clutter,djdeath/clutter-android,spatulasnout/clutter,Distrotech/clutter,kerrickstaley/clutter-vala,GNOME/clutter,dlespiau/clutter,nobled/clutter,spatulasnout/clutter,spatulasnout/clutter,collects/clutter,dlespiau/clutter,djdeath/clutter,jigpu/clutter,spatulasnout/clutter,Distrotech/clutter,djdeath/clutter-multithreaded,ebassi/clutter,jigpu/clutter,djdeath/clutter,GNOME/clutter,Distrotech/clutter,GNOME/clutter,ebassi/clutter,Distrotech/clutter,heysion/clutter-clone,collects/clutter,nobled/clutter,ebassi/clutter,jigpu/clutter,djdeath/clutter-multithreaded,spatulasnout/clutter,nobled/clutter,kerrickstaley/clutter-vala,jigpu/clutter,Distrotech/clutter,heysion/clutter-clone,djdeath/clutter-multithreaded,ebassi/clutter,djdeath/clutter-android,djdeath/clutter-android,djdeath/clutter-multithreaded,ebassi/clutter,djdeath/clutter,djdeath/clutter-android,heysion/clutter-clone,djdeath/clutter-android,djdeath/clutter,GNOME/clutter,collects/clutter,kerrickstaley/clutter-vala