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).
Silence deprecation warning from CMake 3.0+

Setting COMPILE_DEFINITIONS_<Config> has been deprecated since CMake 3.0.
However, the "proper" way of doing things now (generator expressions) have
been support only since CMake 2.8.10, and we require 2.8.9 currently.

Thus, we explicitly enable OLD behavior for CMP0043.