Fixed our main CMakeLists.txt files.

The issue was with CMake Policy #0020 which got introduced in CMake
2.8.11 and which automatically links a qtmain.lib to a Windows
executable. So far, our different solutions ended up working on some
platforms and/or command line / Qt Creator combinations and with CMake
2.8.11 or earlier, but never in all possible combinations. Now, it's
all fine!