COMP: Fix configure error when build against Qt4

This commit fixes the following error:

```
[...]
-- Found Qt4: /home/jcfr/Support/qt-everywhere-opensource-build-4.8.7/bin/qmake (found version "4.8.7")
CMake Error at src/Cxx/Qt/CMakeLists.txt:64 (TARGET_LINK_LIBRARIES):
  Cannot specify link libraries for target "BorderWidgetQt" which is not
  built by this project.
[...]


Former-commit-id: e8c5d86a