make the thing buildable

[codyps]

Thanks!

A few notes:

• the commit revision being fetched by the spec file is fairly old, you may want to update it (ideally, there would be a sane way to automatically update it, but that can wait)

• I'm not sure I'm comfortable doing network access in ./build. Note that including it conditional on ccan/README's existence means that unless the person doing the build is aware of submodules, it could become out of date. Adding it to the instructions seems fine though.

[hramrach]

should be fixed now

[codyps]

Thanks again!