Fix platform detection of OSX ('darwin')

sys.platform returns `darwin` for OSX. As `win` is in `darwin` the old `if` didn't set the `LD_LIBRARY_PATH` on OSX. Unfortunately this fix doesn't solve the issue of not finding `libappleseed.dylib`, when starting `appleseed.cli` or `appleseed.studio` from within Blender.
I will open an issue for this problem.