Support zig cc, have instructions for building outside of devcontainer

[remexre]

It sounds like we could just have zig cc build the right cross-compilation toolchain. The pain of getting the right dependencies then goes way down. We should support this in ./configure and write docs for how to configure it, plus a note that like, you can just use an appropriate host toolchain.

This probably depends on #79, since AFAIU zig cc is always backed by Clang.

For the ./configure interface, maybe we should add a --zig-cc flag that's mutually exclusive with --clang, and makes $CC be ignored in favor of $ZIG, then sets up the appropriate toolchain for the target.