CMake Port: Commercial builds

We need to determine how to enable commercial Qt builds in the future.

Existing functionality

In Qt 5, configure requires the user to choose between opensource and commercial. Either via configure line (-commercial, -opensource) or via a key prompt. Depending on the selection of the user, configure shows the right license.

If commercial was choosen, configure also calls bin/licheck to check for a valid local Qt license.

Finally, subsequent calls of qmake will also periodically run bin/licheck .

CMake port

In the CMake port configure gets replaced by CMake feature files.While cmake does support an interactive mode, it seems the wrong tool for a license check.

Also, the cmake integration currently does not run bin/licheck when user applications are built. Again, whether this is possible at all needs research.