Provide a way to display make's compilation steps for native exts

The problem

I searched for similar problems, sorry if this has been filed already.

Running bundle install --verbose doesn't show the compiler invocation commands for native extensions. I want to be able to see these compiler commands sometimes because I had a build in CI that was taking 20 minutes to install a gem with native extensions, and I didn't know why it was happening. I didn't even know which gem was causing the issue, because gems were being downloaded in parallel and the last output was:

installing gem "some_gem"
However, it was stuck compiling a different gem, further back in the output.

Things I tried

I tried V=1 bundle install as well but it didn't work. If you do make V=1 it normally gives lots of output. I could be doing something wrong though.

The solution

I'm not sure what's a better solution: add a configurable option to bundler or allow certain ENV vars through to the make subprocess.

Start by reproducing the issue with bundle install --verbose on a gem with a native extension, then compare its output with make V=1. The issue is complete when users can identify the gem and see its compiler invocation during installation, through a documented configuration option or supported environment-variable behavior.