[devernay]

fixes #790

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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?

Fix compilation of PythonUtils.cpp with Visual Studio, and implement a more portable version of dirExists()

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

No, as I don't have Windows or VS

[rodlie]

MinGW build OK.