Merge topic 'FindGLUT-pkg-config'

f90d1545 FindGLUT: Use pkg-config to find flags if available
f3f04011 FindGLUT: Reduce if/else nesting and indentation
bece79f9 Tests: Add case covering FindGLUT variables and imported targets

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !6251