Otherwise we have a lot of warnings like:
```
CMake Warning at /usr/lib64/cmake/Qt6/QtPublicWalkLibsHelpers.cmake:284 (message):
When trying to collect dependencies of target
'Constrained_Delaunay_triangulation_2', the non-existent target 'Qt6::Svg'
was encountered. This can likely be fixed by moving the find_package call
that pulls in 'Qt6::Svg' to the scope of directory
'/home/lrineau/Git/cgal-testsuite-dockerfiles/testsuite/CGAL-6.0-Ic-66/test/Triangulation_2_Demo'
or higher. This warning can be silenced by setting
QT_SILENCE_MISSING_DEPENDENCY_TARGET_WARNING to ON.
Call Stack (most recent call first):
/usr/lib64/cmake/Qt6/QtPublicWalkLibsHelpers.cmake:320 (__qt_internal_print_missing_dependency_target_warning)
/usr/lib64/cmake/Qt6Core/Qt6CoreMacros.cmake:609 (__qt_internal_collect_all_target_dependencies)
/usr/lib64/cmake/Qt6Core/Qt6CoreMacros.cmake:709 (_qt_internal_finalize_executable)
/usr/lib64/cmake/Qt6Core/Qt6CoreMacros.cmake:564:EVAL:1 (qt6_finalize_target)
test/Triangulation_2_Demo/CMakeLists.txt:DEFERRED
```
The issue was that `find_package(Qt6) ...` is required in any
sub-directory, because imported targets like `Qt6::Svg` are
local to the directory scope.
```
== CMake setup ==
CMake Warning (dev) at Installation/lib/cmake/CGAL/CGALConfigVersion.cmake:6 (set):
Cannot set "CGAL_MAJOR_VERSION": current scope has no parent.
Call Stack (most recent call first):
CGALConfigVersion.cmake:1 (include)
CMakeLists.txt:13 (include)
This warning is for project developers. Use -Wno-dev to suppress it.
```
When an installed header-only version of CGAL is used, and when all
`demo/*` are configured at once, then only the first demo got
`CGAL_Qt5_FOUND` correctly set.
A lot of parts of CGAL use `<thread>` in conjunction with TBB. I have
added the `find_package(Threads)` in `CGAL_target_use_TBB.cmake` for
convenience.
I have also restored FindTBB.cmake as a pure copy of
https://github.com/Kitware/VTK/blob/master/CMake/FindTBB.cmake
with that version:
> c7249fed2e73ce25fbbc53363d9caf4ab5cc9195
> Author: Cory Quammen <cory.quammen@kitware.com>
> Date: Thu Feb 14 23:21:09 2019 -0500
>
> FindTBB: handle basic linker scripts
>
> TBB binaries may define libtbb.so as a linker script of the form
>
> INPUT(libtbb.so.2)
>
> Make FindTBB find the real .so file libtbb.so points to in such cases.
>
> Approach borrowed from vtk-m/CMake/VTKmDeviceAdapters.cmake on
> suggestion from Robert Maynard.
>
>
Laurent Rineau
Sébastien Loriot
Andreas Fabri
Jane Tournois
Guillaume Damiand
Maxime Gimeno
Simon Giraudot
Jing Yang