Make boost an optional dependency of OpenUSD

[nvmkuruc]

Description of Change(s)

This PR makes boost an optional dependency when not building with examples, python, openvdb, or openimageio. This changes the build_usd script to explicitly create the lib directory with the correct permissions, as downstream builds (like tbb) expected this to have already been created by the boost step.

NOTE on pxr/base/vt/functions.h. This header is unused in OpenUSD outside of python bindings and its test in testVtCpp. It's anticipated that VtCat and its boost dependency will be removed and the changes to pxr/base/vt/CMakeLists.txt will be removed.

This change also anticipates that boost dependencies will be cleaned out of the pch.h files.

Fixes Issue(s)

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• [ ] I have verified that all unit tests pass with the proposed changes
• [x] I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-9507

[jesschimein]

/AzurePipelines run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 1 pipeline(s).

[nvmkuruc]

This no longer depends on the PEGLT PR.