So I’ve been trying to install Jekyll on my Macbook running Mac OS X Yosemite. (The reason I’ve not upgraded is that my version of Parallels, which I use for Windows stuff, insists it won’t run under El Capitan.
I’ve spent two days exploring issues getting the jekyll commands to install - I did have the Mac Developer tools, and I couldn’t work it out. Until I realised I had a really old version of XCode installed on the machine - it claimed to be 4.x.y from memory. A quick trip to the App Store, install the latest version of XCode (over the old version) and bingo it worked.