Nuxi CloudABI is a POSIX-like runtime environment purely built on the
principle of capability-based security[1]. It allows you to run
arbitrary untrusted binaries directly on top of a UNIX kernel without
compromising system integrity.
This change adds a basic platform configuration that defines a small set
of options that allow it to build most of the Boost sources. The next
step is to send out small fixes to individual libraries that don't build
yet.
[1] Nuxi CloudABI: https://github.com/NuxiNL/cloudlibc
