contrib: qt: fix native Qt tools detection

The system may provide matching Qt native tools but the detection and usage of such requires to know the prefix for that native Qt build and the location of its CMake files (which may not be in the prefix, as on Debian).

Now we look for the base Qt in main.mak so the result can be shared between all the Qt targets. All Qt targets share the same "major.minor" value.

Extracted from !5402 (merged)