During the installation of Xcode 4.3 you are given the option to remove previous versions of XCode and the XCode installer. You are warned that the entire /Developer directory will be deleted. One of the hidden implications of this is the loss of various command line tools. The *gcc compiler for example.
After installing Xcode 4.3 on one of my machines I tried to install a different Ruby build using RVM. The install failed since no compiler could be found. Fortunately I had seen an article about Homebrew and the new XCode command line bundle and I rightly figured that installing those tools would allow RVM to complete the installation of new Rubies.
To add the command line tools, open XCode 4.3 (it’s now an application in your /Applications directory) and then open Preferences. Under the Download tab select Components and then click on Install for Command Line Tools.
Update: The Command Line Tools bundle may not be 100% complete. I wasn’t able to use bundler to install the default set of gems for Octopress. A google search on the error led me to the rb-fsevent issues page, where one of the comments suggested using a pre-compiled version of rb-fsevent.
I changed the line in my Gemfile to look like this:
gem 'rb-fsevent', :git => 'git://github.com/ttilley/rb-fsevent.git', :branch => 'pre-compiled-gem-one-off'
and then my bundle install command completed successfully.