FindDevIL: Restore IL_FOUND result variable

The fix in commit v3.8.0-rc1~257^2~1 (FindDevIL: fail properly when
library is not found, 2016-11-24) removed the previously-provided
`IL_FOUND` result variable.  Set it for compatibility and update the
documentation to mention the new variable.

Fixes: #16881
16 jobs for FindDevIL-compat
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