[Feature request] Allow simultaneous build of QT6 and QT5

[werdahias]

It would be nice if the build would allow for simultaneous building of the QT5 and QT6 feature since some applications are already switching to qt6.

[hmaarrfk]

This is somewhat dooable with a little bit of work already, i've been working toward this in:
https://github.com/conda-forge/qgnomeplatform-feedstock/tree/qt6/recipe

You can see the main branch works for qt5, and the branch i liked to qt6 works for qt6. They are in theory co-installable.

[werdahias]

For debian I used two separate builds, one with QT5 and one with QT6.