Fixing gcc on mountain lion for gem install

When you install Mountain Lion, you will no longer have build tools such as `gcc` or `make`

Step 1, get the latest Xcode, and install Command Line Tools, granting you `make` and `gcc`

Step 2, create the symlink so that your bundler/gem commands will locate `gcc` correctly : `sudo ln -s /usr/bin/gcc /usr/bin/gcc-4.2`

Step 3, done! You don’t need MacPorts or a special version of `gcc`, the one installed by Xcode4.5 Developer Preview 2 is just fine and compiles native extensions for me successfully.