Version 4.0.0.

Version 3.7.3.

Version 3.7.2.

Corrected a typo in a comment.

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