Ensure build directory exists before building
Added static_dist target

This creates a "build/static_dist" directory containing the Python
code and a static OCaml binary of 0install. This can be taken to a new
machine an run in place (though you might want to put the directory in
$PATH).
Fixed building of ui.h files on Ubuntu

Reported by Bastian Eicher.
Enable delegating implementation fetching to an external