allow to build s3-crt as static libs without duplicate symbols

[thierryba]

Issue #, if available: https://github.com/aws/aws-sdk-cpp/issues/2842

Description of changes: the s3 and s3crt are defining some common symbols. So when you build as static libs, it does not link due to duplicate symbols.

Check all that applies:

• [X] Did a review by yourself.
• [ ] Added proper tests to cover this PR. (If tests are not applicable, explain.)
• [ ] Checked if this PR is a breaking (APIs have been changed) change.
• [X] Checked if this PR will not introduce cross-platform inconsistent behavior.
• [X] Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• [X] Linux
• [ ] Windows
• [ ] Android
• [X] MacOS
• [ ] IOS
• [ ] Other Platforms

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[jmklix]

Fix merged with this PR: https://github.com/aws/aws-sdk-cpp/pull/2847