This commit is a follow-up of ab02c350 (vtk-config: export OPENGL_GL_PREFERENCE
to the package) where (1) the variable was improperly cased and (2) the
opengl_preference was systematically set to GLVND even if OpenGL::GL was
defined.

This commit implements two key changes:

* updates the variable name to match the name expected by the
  FindOpenGL.cmake module and ensures project depending on VTK will
  look-up the expected OpenGL libraries.

* sets opengl_preference by also explicitly testing the variable "use_legacy"
  Since on some system both set of libraries can be found (GLVND and LEGACY),
  it is critical to explicitly check the user choice.

This is required to successfully configure project like CTK, ITK or VTK
module wheels and ensure the corresponding libraries are not linked
against libOpenGL when OpenGL_GL_PREFERENCE is set to LEGACY.