CMake: Fix issue that led to Windows Unicode support still defaulting to off

[itsmattkc]

Follow-up from #1363

In that PR, a CMake option was introduced called OCIO_USE_WINDOWS_UNICODE to allow users to compile either an ANSI-only or Unicode build on Windows. Its default value was set to WIN32 in an attempt to automatically enable on Windows and disable on other platforms.

It turns out there's a syntax issue here, CMake sets it to the literal string "WIN32" rather than the value of the variable ${WIN32}, and it considers that string literal equivalent to OFF at all times.

This could be corrected by simply changing the default value to ${WIN32} instead of WIN32, which I've tested and works correctly, however what I've written in this PR is a wrapping of that CMake option in an if (WIN32) and defaulting the option toON. Personally, I prefer this approach because I think the code is more readable and there's no real point for a Windows-only option being visible/available on non-Windows platforms anyway. However, if the former approach is preferred, I can write that in instead.

[remia]

Thanks @itsmattkc, I can confirm this works correctly on my Windows box and think it's easier to read that way as well.