gem install cocoapods に 失敗('ruby/config.h' file not found)

少しハマったので備忘録として残しておきます。
## 環境
MacOS : Big Sur 11.1 （20C69）
Xcode : Version 12.3 (12C33)

## 経緯
pod コマンドを使用したところ、cocoapodsをアップデートしてねと言われる

```
% pod outdated
Ignoring ffi-1.13.1 because its extensions are not built. Try: gem pristine ffi --version 1.13.1
Ignoring iStats-1.6.1 because its extensions are not built. Try: gem pristine iStats --version 1.6.1
Updating spec repo `trunk`

CocoaPods 1.10.1 is available.
To update use: `sudo gem install cocoapods`
 :
```

## エラー内容
ほいほいとアップデート実行すると以下のエラーが発生

```
% sudo gem install cocoapods
Password:
Ignoring ffi-1.13.1 because its extensions are not built. Try: gem pristine ffi --version 1.13.1
Ignoring iStats-1.6.1 because its extensions are not built. Try: gem pristine iStats --version 1.6.1
ERROR:  Error installing cocoapods:
	ERROR: Failed to build gem native extension.

    current directory: /Library/Ruby/Gems/2.6.0/gems/ffi-1.13.1/ext/ffi_c
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby -I /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0 -r ./siteconf20210120-82832-1x09kaf.rb extconf.rb
checking for ffi.h... /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:467:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
	from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:585:in `block in try_compile'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:534:in `with_werror'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:585:in `try_compile'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:1109:in `block in have_header'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:959:in `block in checking_for'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:361:in `block (2 levels) in postpone'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:331:in `open'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:361:in `block in postpone'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:331:in `open'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:357:in `postpone'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:958:in `checking_for'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib/ruby/2.6.0/mkmf.rb:1108:in `have_header'
	from extconf.rb:10:in `system_libffi_usable?'
	from extconf.rb:42:in `<main>'

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /Library/Ruby/Gems/2.6.0/extensions/universal-darwin-20/2.6.0/ffi-1.13.1/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Library/Ruby/Gems/2.6.0/gems/ffi-1.13.1 for inspection.
Results logged to /Library/Ruby/Gems/2.6.0/extensions/universal-darwin-20/2.6.0/ffi-1.13.1/gem_make.out
```

`/Library/Ruby/Gems/2.6.0/extensions/universal-darwin-20/2.6.0/ffi-1.13.1/mkmf.log`
上記のエラーログを見てねと言われる。

```/Library/Ruby/Gems/2.6.0/extensions/universal-darwin-20/2.6.0/ffi-1.13.1/mkmf.log
package configuration for libffi is not found
"xcrun clang -o conftest -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/universal-darwin20 -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby/backward -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0 -I. -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT    -g -Os -pipe -DHAVE_GCC_ATOMIC_BUILTINS -DUSE_FFI_CLOSURE_ALLOC conftest.c  -L. -L/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/lib -L. -L/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.Internal.sdk/usr/local/lib     -lruby.2.6   "
In file included from conftest.c:1:
In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby.h:33:
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby/ruby.h:24:10: fatal error: 'ruby/config.h' file not found
#include "ruby/config.h"
         ^~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby/ruby.h:24:10: note: did not find header 'config.h' in framework 'ruby' (loaded from '/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks')
1 error generated.
checked program was:
/* begin */
1: #include "ruby.h"
2:
3: int main(int argc, char **argv)
4: {
5:   return 0;
6: }
/* end */
```
## 対処方法
Xcode の設定(Preferences)から「Locations」タブを開く
「Command Line Tools」が空だったので 「Xcode 12.3(12C33)」を選択

![スクリーンショット 2021-01-20 13.12.30.png](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/812363/0fa661b7-f10c-0d98-ab0a-494b2f7a39c7.png)

上記の設定を行い、再度 `sudo gem install cocoapods`を実行したところ無事インストール完了しました。

#### 参考
https://stackoverflow.com/questions/26434642/yosemite-upgrade-broke-ruby-h