In order to build s6-overlay, you need:

 Hardware:
 - an x86_64 machine (we don't support crossing from other arches yet)
 - more or less 350 MB of disk per arch
   * ~ 250 MB for the toolchain
   * up to 100 MB for all the build artifacts
   * don't worry, the end results are super small
 - a functional network connection to d/l via https and git

 Software:
 - regular POSIX utilities (grep, find, xargs)
 - a cp that supports -a (GNU coreutils or busybox)
 - a sed that supports -i (GNU sed or busybox)
 - GNU make >= 3.81
 - GNU tar with support for .xz archives. We need the --owner options from GNU.
 - git
 - wget or curl or similar fetching command line utility
