Unity build fixes & improvements - Rebased

[rajat2004]

1. Add clean.sh
2. Add autogenerated files and folders to .gitignore (https://github.com/github/gitignore/blob/master/Unity.gitignore)
3. Fix CMake detection, non-standard string comparison, remove whitespace in build.sh
4. Fixes in Unity CMakeLists.txt, add detection of Boost
5. Add files not included in .gitignore
6. Updated assets to Unity 2019 (Will be regenerated if needed for other versions)

I'm not clear about the part of .meta files, would love to have a review of that
Original PR - microsoft#2328
Replaces #5