Heroku ignores kit configuration

I am trying to install the taglib-ruby gem on Heroku. This pearl is compiled as a native extension that requires a system dependency called taglib, so after compiling and downloading it using the heroku volcano, I reached to collect the stone through the command line on heroku bash:

bundle exec gem install taglib-ruby -- --with-opt-dir=/app/vendor/taglib

And in order for this parameter to be used later by bundler , I added it as a bundle configuration via the command:

bundle config build.taglib-ruby '--with-opt-dir=/app/vendor/taglib'

I already checked that this config was applied by checking the /.bundle/config file and looking for the BUNDLE_BUILD__TAGLIB-RUBY line.

However, after clicking my project on the hero and executing the package installation command, heroku complains that the aforementioned stone (taglib-ruby) cannot be installed due to the lack of the taglib library , although this is what I tried to solve using the "- with-opt-dir = / app / vendor / taglib "mentioned above.

So it seems Heroku is ignoring the switch configuration.

What can happen? Do you know another way to achieve the same intention (install a gem with custom build options) on Heroku?