FindOpenGL: Fix confusing output with CMP0072 and OpenGL preference
It can be confusing for some when the LEGACY behavior is selected and the GLVND OpenGL drivers are used anyway (and maybe a little annoying too). This PR fixes that behavior by utilizing a suggestion by Tsyvarev in stackoverflow (https://stackoverflow.com/questions/67685985/cannot-use-legacy-gl-library-with-cmake-ubuntu-18-04-and-20-04).
Topic-rename: FindOpenGL-clarify-output
Edited by Brad King