FindGLUT: Use pkg-config's results as HINTS for normal search

Since commit f90d15458a (FindGLUT: Use pkg-config to find flags if
available, 2021-06-11, v3.22.0-rc1~469^2), pkg-config results are used
directly.  However, this is not compatible with other features of
CMake's find logic such as `CMAKE_FIND_ROOT_PATH` and per-config
results.  Switch to a convention already used by pkg-config support in
other find modules, in which the pkg-config results are only used as
hints for the main search logic.

Fixes: #24028