It's been brought to my attention that the Icecream documentation more or less suggests it is necessary to manually set up $ICECC_VERSION (which also involves creating the environment tarball with the compiler and so on). That is incorrect. I've already updated the documentation to say that, like with pretty much everything, Icecream simply figures it out itself by default.
So if you happen to use $ICECC_VERSION, unless you know why you do that (e.g. cross-compilation), don't. It's not only simpler but also better to leave it up to Icecream to package the system compiler as necessary, as it simply works, and avoids possible problems (such as updating the system compiler and forgetting to update the tarball).