For the CPlusPlus target the target sources contain
"$<TARGET_OBJECTS:3rd_cplusplus>" with older CMake versions.
Looks like that was fixed in CMake 3.14.
Work around it by removing TARGET_OBJECT entries, which we definitely
don't want.
Change-Id: I036720f122aa2d0727e85b611f1e32c65fae57ae
Reviewed-by: Cristian Adam <cristian.adam@qt.io>