【解決方法】Macで gem ffi の インストール (アップデート) に 失敗する

問題

bundle updateを実行した際に、ffi のインストール(アップデート)に失敗した

$ bundle update

# ...(略)

Fetching ffi 1.12.2 (was 1.10.0)
Installing ffi 1.12.2 (was 1.10.0) with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

# ...(略)

extconf failed, exit code 1

Gem files will remain installed in /Users/xxxxx/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/ffi-1.12.2 for inspection.
Results logged to /Users/xxxxx/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/extensions/x86_64-darwin-18/2.6.0-static/ffi-1.12.2/gem_make.out

An error occurred while installing ffi (1.12.2), and Bundler cannot continue.
Make sure that `gem install ffi -v '1.12.2' --source 'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  middleman was resolved to 4.3.6, which depends on
    middleman-core was resolved to 4.3.6, which depends on
      listen was resolved to 3.0.8, which depends on
        rb-inotify was resolved to 0.10.1, which depends on
          ffi

試したこと

ffi をバージョン指定でインストール(アップデート)しようとしても失敗した

$ gem install ffi -v '1.12.2'

Building native extensions. This could take a while...
ERROR:  Error installing ffi:
        ERROR: Failed to build gem native extension.

# ...(略)

解決方法

Xcode Command Line Tool をインストールした上で、ffi をインストール(アップデート)したら成功した

$ xcode-select --install

xcode-select: note: install requested for command line developer tools

$ gem install ffi -v '1.12.2'

Building native extensions. This could take a while...
Successfully installed ffi-1.12.2
Parsing documentation for ffi-1.12.2
Installing ri documentation for ffi-1.12.2
Done installing documentation for ffi after 41 seconds
1 gem installed