warning in Lab

[afabri]

Issue Details

Fix this warning

Source Code

In file included from /home/cgal_tester/build/src/cmake/platforms/Ubuntu-latest-GCC6-CXX1z/test/Lab_Demo/demo_framework_autogen/EWIEGA46WW/../../../../../../../../../../../mnt/testsuite/test/Lab_Demo/Viewer.h:12,
                 from /home/cgal_tester/build/src/cmake/platforms/Ubuntu-latest-GCC6-CXX1z/test/Lab_Demo/demo_framework_autogen/EWIEGA46WW/moc_Viewer.cpp:10,
                 from /home/cgal_tester/build/src/cmake/platforms/Ubuntu-latest-GCC6-CXX1z/test/Lab_Demo/demo_framework_autogen/mocs_compilation.cpp:4:
/mnt/testsuite/include/CGAL/Three/Viewer_interface.h:42:21: warning: defining ‘CGAL::Three::Viewer_interface’, which previously failed to be complete in a SFINAE context [-Wsfinae-incomplete=]
   42 | class VIEWER_EXPORT Viewer_interface : public CGAL::QGLViewer{
      |                     ^~~~~~~~~~~~~~~~
```

[lrineau]

I tried to solve that warning last week, and I concluded (without being completely sure) that it was a compiler bug.

[afabri]

Can we then add an option in the CXX_FLAGS related to -Wsfinae-incomplete ?