Replace WINDOWS with WIN32 in CMakeLists.txt

[acolwell]

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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?

Fixes Windows specific code in CMake files by using the proper platform variable for Windows.

This is a slightly modified version of @cedricp changes proposed in #1040.

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

Built locally and verified Windows specific files were properly included on Windows build.