Qt6Gui considered NOT FOUND on KDE Neon

[SamuXzX]

I followed the "Build and install" steps, but I get a problem when launching inside of the "build" directory:
cmake -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_INSTALL_LIBDIR=lib -DBUILD_TESTING=OFF ..

I get the following error:

CMake Error at /usr/lib/x86_64-linux-gnu/cmake/Qt6Core/Qt6CoreVersionlessTargets.cmake:42 (message):
  Some (but not all) targets in this export set were already defined.

  Targets Defined: Qt::Core

  Targets not yet defined: Qt::CorePrivate

Call Stack (most recent call first):
  /usr/lib/x86_64-linux-gnu/cmake/Qt6Core/Qt6CoreConfig.cmake:72 (include)
  /usr/share/cmake-3.29/Modules/CMakeFindDependencyMacro.cmake:76 (find_package)
  /usr/lib/x86_64-linux-gnu/cmake/Qt6/QtPublicDependencyHelpers.cmake:111 (find_dependency)
  /usr/lib/x86_64-linux-gnu/cmake/Qt6Gui/Qt6GuiDependencies.cmake:42 (_qt_internal_find_qt_dependencies)
  /usr/lib/x86_64-linux-gnu/cmake/Qt6Gui/Qt6GuiConfig.cmake:53 (include)
  /usr/share/cmake-3.29/Modules/CMakeFindDependencyMacro.cmake:76 (find_package)
  /usr/lib/x86_64-linux-gnu/cmake/KDecoration2/KDecoration2Config.cmake:38 (find_dependency)
  CMakeLists.txt:25 (find_package)


CMake Error at /usr/share/cmake-3.29/Modules/CMakeFindDependencyMacro.cmake:76 (find_package):
  Found package configuration file:

    /usr/lib/x86_64-linux-gnu/cmake/Qt6Gui/Qt6GuiConfig.cmake

  but it set Qt6Gui_FOUND to FALSE so package "Qt6Gui" is considered to be
  NOT FOUND.
Call Stack (most recent call first):
  /usr/lib/x86_64-linux-gnu/cmake/KDecoration2/KDecoration2Config.cmake:38 (find_dependency)
  CMakeLists.txt:25 (find_package)


-- Configuring incomplete, errors occurred!

---

I'm on KDE Neon 6.0 with KDE Plasma 6.0.4; QT version is 6.7.0.

[cyrneko]

FYI Lightly does not support Qt6 yet.

see #2