Build issues with Houdini and cmake

**Describe the bug**

Here is a summary of the issues preventing successful building with cmake and houdini:
1. Houdini 21.0: The build fails due to missing shared_lib_targets (usdSkelImaging/hboost_python) in HoudiniConfig.cmake.
2. Houdini 20.5: The build fails with the error cannot find -lhdsi. This appears to be related to the changes introduced in this [commit](https://github.com/Autodesk/arnold-usd/commit/387ab9f984e4b93aaa09a2fba023992de3e1def1#diff-b790d416eae90f3da76049b856f87f7efcb253ccf3ac8de241f783d02a57f7d9).
3. Houdini 20.0: The build also fails with cannot find -lhdsi, stemming from the same commit mentioned above.

Start by reviewing the HoudiniConfig.cmake failures for Houdini 21.0 and the changes in commit 387ab9f984e4b93aaa09a2fba023992de3e1def1 linked in the report. Reproduce the builds for Houdini 21.0, 20.5, and 20.0, then verify that each completes without the missing shared_lib_targets or cannot find -lhdsi errors.

Written by the indexing model from the issue text.