Cmake build: Correct configuration if set LYX_BUNDLE=ON

Cmake build: Correct configuration if set LYX_BUNDLE=ON

Patch by Patrick (pdvisschere@edpnet.be)
Citing:
With some changes I could get it to work (patch included). One will also
have to define the locations of the Qt5-plugins and Qt5-libs folders
(`QT5_PLUGINS_DIR` and `QT5_LIBRARY_DIRS`).

Maybe not all plugins/imageformats are used/needed by lyx, but they are
all copied now, except for `_debug` versions which are excluded.
Starting with Qt-5.12 it seems not possible anymore (or at least not
easy) to link to the `_debug` versions of the qt5-frameworks (with
cmake). And installing them into the bundle also does not work. So right
now this is not a big loss.