This adds modules for the icecream scheduler and daemon.
Icecream can be used for distributed compilation, especially in
environments with diverse toolchains, since it sends the complete build
environment to the daemon.
Unfortunatley the daemon can't be run with DynamicUser = true, since the
daemon requires to be started as root in order to accept other build
environments, see [1].
[1]: https://github.com/icecc/icecream#using-icecream-in-heterogeneous-environments
