This is a simple Heroku buildpack. Having only one simple puprose. That is to seek for a folder .heroku-bundle. If such a file is found what this buildpack will do is it will rename the folder .heroku-bundle to just .bundle. Now, when heroku installs your gems, it will consult this folder and try to find a file /config in it for extra configuration options etc. to use for various gems. For more information on the structure of the file see: This nice gist (there's also a sample config file). If you do not want to manually mess with config file syntax you can refer to the the official documentation and generate the config file locally via bundler commands like bundle config --local build.some_gem "--with-cppflags='-I/app/vendor/prebuilt/include' --with-ldflags='-L/app/vendor/prebuilt/lib'". Obviously, this buildpack is pretty much useless on its own, so should you wish to use it you ought to use it in conjunction with the Heroku Builpack Multi.
For more ideas and a nice tutorial for doing stuff with this (and other related) tool(s), check out this Steve Took's blog article about using charklock_holmes on Heroku.
Example usage:
#Providigin that you have somehow compiled the libraries for the Heroku dynos and copied the prebuilt libraries to vendor/prebuilt...
$ echo >> "gem 'libarchive-ruby'" >> Gemfile
$ bundle
$ heroku run --app cat .bundle/config > .bundle/config #This is to copy the default stuff heroku uses on it's own as the basis...
$ bundle config --local build.libarchive-ruby "--with-cppflags='-I/app/vendor/prebuilt/include' --with-ldflags='-L/app/vendor/prebuilt/lib'"
$ mv .bundle .heroku-bundle #You do not want to use this version of the gem locally.. only on heroku
$ git add .heroku-bundle Gemfile Gemfile.lock
$ git commit -m "Added libarchive-ruby gem."
$ git push heroku master
...
-----> Heroku receiving push
-----> Fetching custom buildpack
-----> Heroku Bundle Config app detected
-----> Found a .heroku-bundle folder
The buildpack will detect that your app has a .heroku-bundle folder in the root app directory. This folder will be renamed to .bundle. If a .bundle folder already exists it will be deleted.