we use `#include <QGLViewer/qglviewer.h>`, so `QGLViewer_INCLUDE_DIR`
must not include `QGLViewer` component.

Also, on darwin, headers are installed in Headers dir, not include, ref:
https://github.com/GillesDebunne/libQGLViewer/blob/ba9a875784afbb7ee73088fe0e8701c31bc7277d/QGLViewer/QGLViewer.pro#L138

This require fixes in upstream nixpkgs:
NixOS/nixpkgs#348549

So we can use the source of that PR for now

ref. https://github.com/GillesDebunne/libQGLViewer/blob/ba9a875784afbb7ee73088fe0e8701c31bc7277d/QGLViewer/QGLViewer.pro#L176

without this, `lib/cmake/Sofa.GUI.Qt/Sofa.GUI.QtConfig.cmake` has:
> `set(SOFA_GUI_QT_HAVE_QT6 0)`

and therefore, in SofaPython3, `find_package(QGLViewer QUIET REQUIRED)`
is not called, and build ends up with:
> [ 98%] Linking CXX shared library ../../lib/python3/site-packages/Sofa/Gui.cpython-312-x86_64-linux-gnu.so
> […]/ld: cannot find -lQGLViewer: No such file or directory

TODO: This should be fixed in CMake instead, but I have no clue.

error was:
> dyld[61665]: Library not loaded: /nix/store/aalbn4pznxwy18ydjmb15c3y4izj8isi-sofa-24.06.00/lib/libSceneChecking.24.12.99.dylib
>   Referenced from: <8B75C775-7FE5-3755-A190-B11A3F4F6666> /nix/store/aalbn4pznxwy18ydjmb15c3y4izj8isi-sofa-24.06.00/bin/.runSofa-24.12.99-wrapped
>   Reason: tried: '/nix/store/aalbn4pznxwy18ydjmb15c3y4izj8isi-sofa-24.06.00/lib/libSceneChecking.24.12.99.dylib' (no such file), '/System/Volumes/Preboot/Cryptexes/OS/nix/store/aalbn4pznxwy18ydjmb15c3y4izj8isi-sofa-24.06.00/lib/libSceneChecking.24.12.99.dylib' (no such file), '/nix/store/aalbn4pznxwy18ydjmb15c3y4izj8isi-sofa-24.06.00/lib/libSceneChecking.24.12.99.dylib' (no such file), '/usr/local/lib/libSceneChecking.24.12.99.dylib' (no such file), '/usr/lib/libSceneChecking.24.12.99.dylib' (no such file, not in dyld cache)
> zsh: abort      ./result/bin/runSofa