Auto merge of #5933 - NickLaMuro:bundle-pristine-respect-bundler-config, r=segiddins

Use Bundler::Installer for bundle pristine

Fixes bundler pristine to respect the `bundle config` options for particular gems.  In my particular case, on OSX I have a keg version of `pg` and a bundle config with the following entry:

```console
$ bundle config
...

build.pg
Set for the current user (/Users/nicklamuro/.bundle/config): "--with-pg-config=/usr/local/Cellar/postgresql@9.5/9.5.6/bin/pg_config"

```

This would not be respected

What was the end-user problem that led to this PR?
--------------------------------------------------

See above, but steps to reproduce (requires `bundler` >= 1.15):

1. Install postgres on your machine if it is not already there (OS dependent)

2. Add the following bundler build arg for `pg`:

  ```console
  $ bundle config build.pg -- --with-pg-config=$(which pg_config)
  ```

2. Make sure `pg_config` is not in your `$PATH`:

  ```console
  $ export PATH=$(echo "$PATH" | sed "s|:$(dirname $(which pg_conf)))||")
  ```

* Create a simple project with `pg` as a single gem in your Gemfile:

  ```
  # Gemfile
  gem "pg"
  ```

* Bundle:

  ```console
  $ bundle install
  ```

* Attempt to `bundle pristine` (you should get an error):

  ```console
  $ bundle pristine
  ```

What was your diagnosis of the problem?
---------------------------------------

The newly added `bundle pristine` did no use the same code that was used by `bundle install` to execute the re-installation of the gem.

What is your fix for the problem, implemented in this PR?
---------------------------------------------------------

By making use of the `Bundler::Installer` and `Bundler::Installer::GemInstaller` code that is already used with `bundle install`, we can reuse the code that injects the `bundle config` options into each gem that is being installed.

Why did you choose this fix out of the possible options?
--------------------------------------------------------

Didn't want to repeat code that was already being used elsewhere.  Caused a few lines of code to be added that weren't there previously, but nothing obscene.

1 files changed, 8 insertions, 1 deletions

diff --git a/spec/support/builders.rb b/spec/support/builders.rb
index af91c5e6a7..93b173a5d0 100644
--- a/spec/support/builders.rb
+++ b/spec/support/builders.rb
@@ -563,10 +563,17 @@ module Spec
         write "ext/extconf.rb", <<-RUBY
           require "mkmf"
 
+
           # exit 1 unless with_config("simple")
 
           extension_name = "very_simple_binary_c"
-          dir_config extension_name
+          if extra_lib_dir = with_config("ext-lib")
+            # add extra libpath if --with-ext-lib is
+            # passed in as a build_arg
+            dir_config extension_name, nil, extra_lib_dir
+          else
+            dir_config extension_name
+          end
           create_makefile extension_name
         RUBY
         write "ext/very_simple_binary.c", <<-C