On a very brief glance I can't see an obvious reason why that is not being built.
However, Akonadi 16.12, which we intend to ship with zesty, on a rebuild against Qt 5.7.1 successfully builds and installs the sqlite backend library.
The sqlite/postgresql backend are also more or less depreciated or at least problematic AFAIK, and kubuntu has as a result set a depends just on the mysql backend.
On a very brief glance I can't see an obvious reason why that is not being built.
However, Akonadi 16.12, which we intend to ship with zesty, on a rebuild against Qt 5.7.1 successfully builds and installs the sqlite backend library.
The sqlite/postgresql backend are also more or less depreciated or at least problematic AFAIK, and kubuntu has as a result set a depends just on the mysql backend.