Summary
Make code and build changes to enable running S3 extension on OS X.

Motivation
Some effort was done as part of WT-8720 to get the extension compiling and working on OS X. There is a PR linked to that ticket and some comments detailing where the efforts were. Continue the work and get the extension working on OS X. The extension could be compiled and tested on OS X 10.15, but not 10.14 and WT's evergreen project only uses 10.14. Alternative code changes to WT-8720 could be explored, or to add to the continuous testing.

Note:
Do consider the costs involved in interacting with S3. Aim for a balance between the costs and benefits.

• Is this issue urgent?
  No

Acceptance Criteria (Definition of Done)
The extension builds and tests on OS X. There was some effort in figuring out if and how we can test the extension on OS X in evergreen.