CMake UNIX Platform Fix

[bedwardly-down]

This simple commit could solve some problems later down the road. In at least newer CMake versions, checking if the platform is UNIX like you had can cause Apple and Cygwin targets to be a miss since both count as UNIX in CMake.

See here for more information: https://cmake.org/cmake/help/latest/variable/UNIX.html

[HappySeaFox]

Hi! Thanks for the PR! From my understanding, the result of this long block of "ifs" will be the same on Apple or Cygwin platforms even after merging this PR. Both the SAIL_APPLE and SAIL_UNIX will still be set on Apple platforms. So the PR doesn't alter the current behavior. Please correct me if I'm wrong.

The whole purpose of this block of "ifs" is to mimic the CMake behavior without introducing any new logic. Since on Apple platforms both the APPLE and UNIX are defined, SAIL does the same 🙂

[bedwardly-down]

Right now, as is, it should be fine. Moving that to the bottom is more for readability than anything. If you change something down the line that requires one or the other being defined, though, it could help with debugging.

UNIX is a catch all for all non-Windows, non-mobile OS's / platforms (even the PS4 and PS5 count as Unix due to being built on modified BSD OS's). 😸