5.5 deb packages fail to be created on Trusty due to absence of OQGRAPH

After the fix for MDEV-4795, server can be built on a system with a wrong boost version, and this is good.
However, the debian packages still fail to build, because OQGRAPH is absent while it is expected. As a result, we don't provide 5.5 packages for Trusty at all, and it is getting bad since Trusty is going GA soon.

See for example http://buildbot.askmonty.org/buildbot/builders/kvm-deb-trusty-x86/builds/35/steps/compile/logs/stdio

Either we need to downgrade the VMs, or make ha_oqgraph optional (if it's possible).