CMake: Don't link directly to libGL on Unix.

(we always dynamically load it at runtime when appropriate.)

cmake/sdlchecks.cmake

@@ -679,7 +679,6 @@ macro(CheckOpenGLX11)
       set(SDL_VIDEO_OPENGL 1)
       set(SDL_VIDEO_OPENGL_GLX 1)
       set(SDL_VIDEO_RENDER_OGL 1)
-      list(APPEND EXTRA_LIBS GL)
     endif()
   endif()
 endmacro()