This is a workaround for issue #5.

We also remove the `CONFIG += link_pkgconfig` from `engine.pro` because
this directive is already set in `config.pri`.

Some feature from C++11 is used (at least one "nullptr" was found in the
source code).

After this change, the engine can be built only with qt5 and qtpim.

Also switch to using sqlite backend for all tests, and remove some
irrelevant testing code.