Using symbol __unix__ to improve portability

[vtamara]

Replaces https://github.com/ONLYOFFICE/core/pull/66

[ShockwaveNN]

Sorry for late response. According to https://stackoverflow.com/questions/7063303/macro-unix-not-defined-in-macos-x __unix__ is not available on MacOSX and iOS, so we think it may cause trouble in some cases for your changes (in some of changed lines - there is already check for macos, but I think not at all cases)

[lugeluge]

This project is great. I wonder if I can compile it on vs2017？

[CLAassistant]

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