Borrow Rust's quiet trick to have toggable silenced output.

Run with `VERBOSE=1` to see the commands (e.g. `make VERBOSE=1`).

Fixes #104
Fixes #107
DEFAULT should be a dependency not a recursive make invocation.

This avoids the ugly output like

    make help
    make[1]: Entering directory `...`
    ...
    make[1]: Leaving directory `...`
Define the rusti script with `define` to avoid an ugly string literal.

Fixes #109