MACでvagrant-vbox-snapshotのプラグインを入れようとしたらffi (1.9.6)のエラーが出た

MACでvagrant-vbox-snapshotのプラグインを入れようとしたら以下のエラーが。

```
$ vagrant plugin install vagrant-vbox-snapshot
Installing the 'vagrant-vbox-snapshot' plugin. This can take a few minutes...
Bundler, the underlying system Vagrant uses to install plugins,
reported an error. The error is shown below. These errors are usually
caused by misconfigured plugin installations or transient network
issues. The error from Bundler is:

An error occurred while installing ffi (1.9.6), and Bundler cannot continue.
Make sure that `gem install ffi -v '1.9.6'` succeeds before bundling.

Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

    /Applications/Vagrant/embedded/bin/ruby extconf.rb 
checking for ffi.h... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
	--with-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
	--with-make-prog
	--without-make-prog
	--srcdir=.
	--curdir
	--ruby=/Applications/Vagrant/embedded/bin/ruby
	--with-ffi_c-dir
	--without-ffi_c-dir
	--with-ffi_c-include
	--without-ffi_c-include=${ffi_c-dir}/include
	--with-ffi_c-lib
	--without-ffi_c-lib=${ffi_c-dir}/
	--with-libffi-config
	--without-libffi-config
	--with-pkg-config
	--without-pkg-config
/Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:434:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
	from /Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:549:in `block in try_compile'
	from /Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:502:in `with_werror'
	from /Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:549:in `try_compile'
	from /Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:1044:in `block in have_header'
	from /Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:895:in `block in checking_for'
	from /Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:340:in `block (2 levels) in postpone'
	from /Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:310:in `open'
	from /Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:340:in `block in postpone'
	from /Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:310:in `open'
	from /Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:336:in `postpone'
	from /Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:894:in `checking_for'
	from /Applications/Vagrant/embedded/lib/ruby/2.0.0/mkmf.rb:1043:in `have_header'
	from extconf.rb:16:in `<main>'


Gem files will remain installed in /Users/yonezawas/.vagrant.d/gems/gems/ffi-1.9.6 for inspection.
Results logged to /Users/yonezawas/.vagrant.d/gems/gems/ffi-1.9.6/ext/ffi_c/gem_make.out
```

gemでfiiを入れろと書いてありますが、sudoで実行してもエラーは解消せず。

以下のコマンドで解消できます。

```
sudo gcc
```

ライセンス表示が出るのでそこは表示で。

下の画面で「**agree**」を入力すると実行が進みます。

```
By typing 'agree' you are agreeing to the terms of the software license agreements. Type 'print' to print them or anything else to cancel, [agree, print, cancel] agree

You can view the license agreements in Xcode's About Box, or at /Applications/Xcode.app/Contents/Resources/English.lproj/License.rtf

clang: error: no input files
```

その後、プラグインのインストールはうまくいきまいた。

```
$ vagrant plugin install vagrant-vbox-snapshot
Installing the 'vagrant-vbox-snapshot' plugin. This can take a few minutes...
Installed the plugin 'vagrant-vbox-snapshot (0.0.8)'!
```