Fix 'unknown warning option' warnings when building with Clang

[acolwell]

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• I've followed the contributing guidelines to the best of my understanding
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PR Description

What type of PR is this? (Check one of the boxes below)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• Breaking change (fix or feature that would cause existing functionality to change)
• My change requires a change to the documentation
  • I have updated the documentation accordingly

What does this pull request do?

Updates cmake build file to only specify gcc specific warnings when gcc is being used to build the code.

Have you tested your changes (if applicable)? If so, how?

Yes. I built the code locally with gcc and with clang. I verified that the clang builds no longer complain about unknown options and the options are still specified on gcc builds.

[YakoYakoYokuYoku]

Would've added in the PR post body that it fixes #1027, LGTM regardless.