Fix SSL detection

As there is no easy way to check for QT_CONFIG with Qt5 and CMake,
and the way we did it for Qt4 is hackish at best, let's do it the
proper way and compile a short code snippet that checks for the
appropriate defines (QT_NO_OPENSSL for Qt4, and QT_NO_SSL for Qt5).

This should be the most robust way of figuring this out, and is rather
easy with CMake.