Tom Fogal authored Jun 08, 2017 and Brad King committed Sep 25, 2017

Find GLVND components if available.  Add `GLX` and `EGL` options for
COMPONENTS that allow requesting these libraries explicitly.  Introduce
new import targets for these windowing-system-specific libraries.

On a GLVND system, populate the legacy `OPENGL_LIBRARIES` variable and
the `OpenGL::GL` target using the `OpenGL` and `GLX` components.  On
non-GLVND systems, continue to use the legacy `GL` library and simply do
not provide the GLVND components.  Application code can choose to adapt
based on the availability of GLVND components as imported targets.