[seL4] Demo build

Most of my experience with the seL5 microkernel has been through the Genode framework, but now I am attempting to build the demo test of seL4 without Genode using the instructions on the "Getting Started" page of seL4.systems but this seems to not be working for me. I have the latest version of repo, ninja and CMake (originally, I was given an error that I needed 3.9.x so I upgraded CMake manually). When I get to the instruction of "../init-build.sh -DPLATFORM=x86_64 -DSIMULATION=TRUE" demo build, the script only hangs. Nothing gets built and no demo is run. Is there something missing that I should be doing?

Thanks, Chris