FindOpenGL: Add option to prefer GLVND for legacy GL

Since !1466 (merged) users may set OPENGL_gl_LIBRARY to empty to use GLVND components for the legacy GL interfaces. This is useful only when one knows in advance that the GLVND components will be found.

Add a OpenGL_GL_PREFERENCE variable to specify a preference for legacy GL or GLVND. The latter can suppress OPENGL_gl_LIBRARY only when the needed GLVND components are found. If no preference is explicitly specified, choose a default based on whether GLVND components were requested (because this indicates the project has been updated for CMake 3.10).

Issue: #17437 (closed)
Issue: #17449 (closed)