I want to install Octopress in my computer.
I try to do it like
Octopress document.
When I run
bundle install
I was installing Octopress too and got the same problem as yours. After checking the mkmf.log file I figured out that gcc-4.2 had not been found. I Even installed xcode with the command line tools. It worked when I made a symlink like this:
$ sudo ln -s /usr/bin/gcc /usr/bin/gcc-4.2
$ bundle install