Railsアプリ（かなり古いバージョン）でjsonエラーが発生し、他の時点でmysqlエラーが発生したため、rvmを使用してすべてを最新バージョンに再インストールしました。私がグーグルしているものの多くは古いバージョンのxcodeを言っていますが、実行すると次のxcodebuild -versionようになります。
Xcode 4.5.2
Build version 4G2008a
geminstallrailsの完全なエラーは次のとおりです。
jakes-MacBook-Pro:~ mctesterson$ sudo gem install rails
Fetching: i18n-0.6.1.gem (100%)
Fetching: multi_json-1.4.0.gem (100%)
Fetching: activesupport-3.2.9.gem (100%)
Fetching: builder-3.0.4.gem (100%)
Fetching: activemodel-3.2.9.gem (100%)
Fetching: rack-1.4.1.gem (100%)
Fetching: rack-cache-1.2.gem (100%)
Fetching: rack-test-0.6.2.gem (100%)
Fetching: journey-1.0.4.gem (100%)
Fetching: hike-1.2.1.gem (100%)
Fetching: tilt-1.3.3.gem (100%)
Fetching: sprockets-2.2.2.gem (100%)
Fetching: erubis-2.7.0.gem (100%)
Fetching: actionpack-3.2.9.gem (100%)
Fetching: arel-3.0.2.gem (100%)
Fetching: tzinfo-0.3.35.gem (100%)
Fetching: activerecord-3.2.9.gem (100%)
Fetching: activeresource-3.2.9.gem (100%)
Fetching: mime-types-1.19.gem (100%)
Fetching: polyglot-0.3.3.gem (100%)
Fetching: treetop-1.4.12.gem (100%)
Fetching: mail-2.4.4.gem (100%)
Fetching: actionmailer-3.2.9.gem (100%)
Fetching: rake-10.0.2.gem (100%)
Fetching: rack-ssl-1.3.2.gem (100%)
Fetching: thor-0.16.0.gem (100%)
Fetching: json-1.7.5.gem (100%)
Building native extensions. This could take a while...
ERROR: Error installing rails:
ERROR: Failed to build gem native extension.
/Users/mctesterson/.rvm/rubies/ruby-1.9.3-p327/bin/ruby extconf.rb
creating Makefile
make
compiling generator.c
make: /usr/bin/gcc-4.2: No such file or directory
make: *** [generator.o] Error 1
Gem files will remain installed in /Users/mctesterson/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/gems/1.9.1/gems/json-1.7.5 for inspection.
Results logged to /Users/mctesterson/.rvm/rubies/ruby-1.9.3-p327/lib/ruby/gems/1.9.1/gems/json-1.7.5/ext/json/ext/generator/gem_make.out