fixes to build using cmake on macOS/Qt5

[devernay]

Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. Additionally, make sure you've done all of these things:

• I've followed the contributing guidelines to the best of my understanding
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• I've formatted my code according to Natron's code style
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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?

Add support for building using cmake on macOS / homebrew.

Building on macOS / MacPorts will be more complex as it requires modifying MacPorts's pyside install, which doesn't include cmake or pkg-config files.

[YakoYakoYokuYoku]

Otherwise LGTM.

[YakoYakoYokuYoku]

Only things that remain are those requested changes and I'd squash the commits of this PR into one or two.

[devernay]

commits from one PR are usually squashed anyway.