Updated CMake minimum version to 3.13.

Renamed HEXL to hexl in pkgconfig.

Edited CHANGES.md.

Removed installation of clang-9 from nix pipeline.

Merge pull request microsoft#322 from g2flyer/hexl-install-fix

fix of hexl install bug

Copy NOTICE file to drop

Edited CHANGES.md

Switched to version 3.6.1

Edit to CHANGES.md

Switch to 3.5.9