Lab: Suppress warning

[afabri]

Summary of Changes

Suppress warning issued by gcc.

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{
      |                     ^~~~~~~~~~~~~~~~

Release Management

• Affected package(s): Lab
• License and copyright ownership: unchanged

[afabri]

The warning is gone for the test platform mentioned above, but it triggers a new warning on other gcc plarforms

include/CGAL/Three/Viewer_interface.h:37:34: warning: unknown warning group '-Wsfinae-incomplete', ignored [-Wunknown-warning-option]
#  pragma GCC diagnostic ignored "-Wsfinae-incomplete"

Either we refine the conditions of the #ifdef or we at the same time add the -Wunknown-warning-option.

[sloriot]

Generate this warnings on several platforms:

/Users/geometryfactory/CGAL/testsuite/CGAL-6.2-Ic-137/include/CGAL/Three/Viewer_interface.h:37:34: warning: unknown warning group '-Wsfinae-incomplete', ignored [-Wunknown-warning-option]
#  pragma GCC diagnostic ignored "-Wsfinae-incomplete"

[sloriot]

Successfully tested in CGAL-6.2-Ic-150