FindGLUT: Use link directories for libs from pkg-config

Since commit f90d1545 (FindGLUT: Use pkg-config to find flags if
available, 2021-06-11, v3.22.0-rc1~469^2), the `GLUT_LIBRARIES` may come
from `pkg-config`, in which case they may be `-l` flags instead of
absolute paths.  Propagate `GLUT_LIBRARY_DIRS` to GLUT::GLUT too.

Fixes: #23229
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