Adapt CMake logic for finding lconvert in newer Qt5

Seems like Qt 5.3 has added the long-missing lconvert target in its CMake
support scripts, however it didn't also add the Qt5_LCONVERT_EXECUTABLE
variable to go along with that. Adding the target disabled our workaround,
breaking translation support in Quassel.

We now also cover this case (and made things future-proof in case Qt5 ever
adds the variable upstream).