We've been building release tarballs with the Autotools for both libmongoc and libbson. Port this feature to CMake. Let's only build a release tarball for libmongoc from now on: we had already included a bundled libbson in libmongoc's release tarball, so we can stop publishing a tarball for libbson now.

The contents of the libmongoc release tarball built by CMake should be nearly the same as the one we used to build with the Autotools. Some files will disappear:

• libbson had its own copies of LoadVersion.cmake and MaintainerFlags.cmake in src/libbson/build/cmake, those are now deleted because they're copies of libmongoc's files
• all the Autotools scripts will be deleted

The check-release-archive.sh script in the repository is run on Evergreen to build the release archive with the Autotools. It runs "make distcheck" and also runs additional checks. This script should build a CMake target instead of running "make distcheck", and it should apply the same set of checks to files included from libbson as it already does to files included from libmongoc.