My system has:
I ran into this problem too. Running the following installed Jekyll for me:
$ \curl -L https://get.rvm.io | bash -s stable --rails --autolibs=enabled
$ sudo gem install jekyll
This installs RVM, updates Rails and installs Jekyll.
BTW - Most of the posts I've found indicate it's a problem with Xcode not having the developer tools installed. I have a newer MacBook Pro on which this stuff was installed by default, so attempting to install the Xcode dev tools did nothing for me (unlike the above, which did everything I needed).