Can't compile on libGLVnd systems: Can't find OpenGL::GL

[ManDay]

C.f. #6050

The linked PR doesn't work for libGlVnd system. I think we need an "IF" to use either the one (::OpenGL) or the other (::GL).

Documented (kind of inaccurately, hence the mistake?) in /usr/share/cmake/Modules/FindOpenGL.cmake