Fixes for building against usd-0.19.3

[pmolodo]

Description (this won't be part of the changelog)

Fixes for building against usd-0.19.3

Changelog

Fixed

• usd-0.19.3 fix: filesystem library needs system lib - need to explicitly link now that usd doesn't

Checklist (Please do not remove this line)

• [X] Make sure the Title and Description of the PR make sense and provide sufficient context for your work
• [X] Do any added files have the correct AL Apache Licence Header?
• [X] Are there Doxygen comments in the headers?
• [X] Are any new features, behaviour changes documented in the .md format documentation?
• [X] Have you added, updated tests to cover new features and behaviour changes?
• [X] Have you filled out at least one changelog entry?
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[pmolodo]

Bit late of a response, but...

I don't think there's anything new that's using system. The issue is just that AL_USDMaya uses the boost filesystem lib, and that in turn uses the system lib. Previously, though, the various pixar cmake machinery used to automatically add in the "system" lib into the list of boost dependencies. It no longer does that (I guess they're not using it anymore, or just using it in fewer places?), so AL now needs to explicitly link against it, instead of relying on the pixar cmake to do it "for us".