a) using a system provided KF6
b) downloading and compiling KF6
c) using the vendored-in re-creation of the functionality
The behaviour for both Qt5 and Qt6 is steered with the same two variables which were renamed to become version agnostic:
a) BUILD_WITH_KF=ON BUILD_KF=OFF
b) BUILD_WITH_KF=ON BUILD_KF=ON
c) BUILD_WITH_KF=OFF
The version is automatically derived from the chosen Qt major version.
Those desktop MIME types were needed only with KDE up to 3.x, as it
used to have its own desktop-based MIME type system. KDE 3 is EOL for
many years now, and there are already XDG MIME types.
These files are the bare XML definitions, and they are automatically
generated by update-mime-database (part of shared-mime-info) on update
(e.g. by distro hooks) or manually. Keeping them in the sources, and
installing them, is definitely not correct, as qelectrotech.xml is
their canonical definition.
Hence, drop them from the sources, together with references to them.
Andre Rummler