How to specify env variable while installing a gem trough bundler?

I am trying to approach ‘TDD Spec Cleanup Exercise on Fundamentals’ of ‘TDD trail’. The exercise requires me to have Qt in order to compile capybara-webkit. And it happened so that I’m using MacOS Sierra.
I was lucky enough to find this guide in thoughtbot’s repository, which explains how to install Qt under my circumstances. It is my understanding that I need to append another value to my PATH like so:

PATH=$PATH:/Users/your_user_name/Qt5.5.1/5.5/clang_64/bin gem install capybara-webkit

Unfortunately, this guide doesn’t take into account that I’m using bundler to manage dependencies and I’d like to use bundler’s own option bundle.config to tune installation process of a specific gem.
I wasn’t able to find instructions on my particular desire, so could anyone give me guidance on this matter?

How do I tune environment variables for installation a bundler managed gem?